Draft (Jan. 18, 2010) Executive Summary




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4.6.2Searching for first stars and cosmic stellar relics in the Galaxy


Haining Li (NAOC), Gang Zhao (NAOC)
Key questions:


  1. How many metal-poor stars with [Fe/H] < -5.0 are there in the Galaxy?

  2. Are there really two components of the Galactic halo?

  3. Is there any first generation of stars or its relics in the Galaxy?


Introduction:
Metal-poor stars with atmospheric abundances of the heavy elements (such as iron) that are substantially lower than the solar content provide fundamental constraints on numerous issues of contemporary interests, from the nature of the Big Bang, to the astrophysical sites of neutron-capture element production. Specially, extremely metal-poor stars provide important clues to the chemical history of our Galaxy, the role and type of early supernovae, the mode of star formation in the proto-Milky Way, and the formation of the Galactic halo. Automatic spectral classification and processing pipelines have made large scale stellar spectroscopic survey possible, and have already obtained many metal-poor star candidates [1-3]. However, it is only possible, with high-resolution spectroscopic observation by telescopes like TMT, that we can determine accurate elemental abundances and chemical patterns to understand their nature, especially for stars with metallicity [Fe/H] < -3.0.
Main science areas:
Searching for metal-poor stars
Stars with metallicities below 1/1000 of that of the Sun, i.e., [Fe/H] < -3.0, are referred to extremely metal-poor stars (EMP stars), and are regarded the local relics of epochs otherwise only observable at high redshifts [4]. These stars preserve, to a large extent, the chemical and kinematic signatures of the gas clouds from which they formed, and hence they are important for studying the formation and chemical evolution of the Galaxy, the properties (e.g., mass, rotation) of the first generation of massive stars which exploded as type II supernovae (SN II), nucleosynthesis and star formation processes in the early Universe. Age determinations of metal-poor, old stars also provide a lower limit for the age of the Universe [5].
Objective-prism or moderate-resolution (R ≤ 2000) spectroscopic and photometric survey, such as HK, HES, SDSS/SEGUE, etc., have been quite successful in searching for metal-poor stars and systematic researches including the tomography of the Milky Way and the metallicity distribution function of the Galactic halo, as well as in collecting possible candidates for EMP stars. On the one hand, due to their limited accuracy in metallicity measurements and elemental analysis, moderate-resolution spectroscopic observations are unreliable in identifying or analyzing stars with [Fe/H] < -3.0, for which follow-up high-resolution observations become rather crucial for constructing the metal-deficient “tail” of the metallicity distribution function of Galactic halo [6]. On the other hand, there are still rather limited high-resolution spectra available for metal-poor star candidates; only two stars with [Fe/H] < -5.0 have been detected so far, although several 8m telescopes are currently involved in follow-up observations for metal-poor star candidates.
Therefore extremely large telescopes such as TMT are an unprecedented tool in identification and analysis of metal-poor star candidates with [Fe/H] < -3.0, and in the future discovery of hyper metal-poor stars with [Fe/H] < -5.0 or even -6.0, which would undoubtedly help us explore the first generation of stars and the very beginning of Galactic chemical evolution.

Possible TMT programs:

  1. TMT WFOS/HROS follow-up observations of metal-poor star candidates with [Fe/H] < -3.0 identified by the LAMOST metal-poor star survey. Such a more complete metal-poor star sample, combined with reliable kinematical measurements, will enable a systematic analysis on the components of Galactic halo.

  2. A special project with WFO/HROS aimed at finding metal-poor stars with peculiar chemical patterns related to Population III (e.g., pre-enrichment by pair-instability SNe, etc.).


China’s strengths and weakness in this area:

We propose to carry out a new survey for metal-poor stars with LAMOST covering ~5500 deg2 of the northern high-galactic latitude (|b| > 45°) sky down to B = 18.5 mag. Using the low-resolution (R = 2000) LAMOST spectra in combination with SDSS photometry, we will determine the stellar parameters Teff, log g and [Fe/H] of about 2,500,000 stars to identify the most metal-poor candidates among them. Together with follow-up high-resolution observations, we expect to greatly increase the total number of known metal-poor stars, including the discovery of ~1200-1300 new extremely metal-poor stars, and ~12-13 hyper metal-poor stars.


Such a large scale survey needs to compete against a number of mature surveys including SEGUE, it is thus important to have an optimal strategy for the LAMOST metal-poor star survey and follow-up observations that require coordination with these existing projects.
References:

  1. Beers, T. C., Rossi, S., O’Donoghue, D., et al. 1999, in Astronomical Society of the Pacific Conference Series, Vol. 165, The Third Stromlo Symposium: The Galactic Halo, ed. B. K. Gibson, R. S. Axelrod, & M. E. Putman, 254

  2. Christlieb, N. 2003, in Reviews in Modern Astronomy, Vol. 16, Reviews in Modern Astronomy, ed. R. E. Schielicke, 191

  3. Carollo, D., Beers, T. C., Lee, Y. S., et al. 2007, Nature, 450, 1020

  4. Beers, T. C., & Christlieb, N. 2005, ARA&A, 43, 531

  5. Frebel, A., Aoki, W., Christlieb, N., et al. 2005, Nature, 434, 871

  6. Shoerck, T., Christlieb, N., Cohen, J. G., et al. 2008, ArXiv e-prints


4.6.3The merging history of the Galaxy and stellar abundances of Galactic globular clusters


Yuqin Chen, Gang Zhao, Jingkun Zhao (NAOC), Huawei Zhang (PKU)
Key questions:

  1. What is the merging history of the Galaxy?

  2. What are the abundance patterns of stars in Galactic globular clusters?


Introduction:
Mergers are expected to be a common phenomenon during the formation of galaxies. Mergers are expected to have left behind a large amount of substructure in the phase-space distribution and kinematics of stars, especially in the stellar halo, and probably also in the thick disc and bulge of our Galaxy. As the Galaxy ages, the grouping phenomena in density or kinematics space overlap more and more, and thus it is difficult to detect them in old streams and moving groups now. But their member stars will be identifiable by their chemical imprints.
Some globular clusters may also have been driven by merger events, which may be extremely common at the early time. The origin of the globular clusters is still rather unclear and but likely involves connections with all the three other components of the Galaxy. The identification of their origin from the abundance patterns of their stars in globular clusters would help to understand their link with the thick disc, and their detailed elemental abundances will provide further information on the diffusion theory for stellar astrophysics.
It is clear that nearby dwarf galaxies contribute to the formation of the Galactic halo, but halo field stars have different compositions to the surviving satellites. Observations and modeling of such differences may provide insights into the nature of satellites that may previously have been stripped, such as their stellar and dark-matter masses, densities, spatial distributions and angular momentum, compositions and kinematics.


Main science areas:
The evidence of merging history of the Galaxy
There are many stellar streams, such as Sagitarrius tidal trail, the Monoceros ring, etc, detected outside the solar circle. The stars in streams are believed to have formed outside the Galaxy, via the merging with nearby galaxies. The detailed kinematics and abundances of stars down to main sequences of the tidal tails are not well studied with 8-10m telescope. It is interesting to obtain the abundance patterns of dwarf stars in these stellar streams with TMT in order to check if they have the similar origins in chemical space. In the solar neighbourhood, the Hercules stream associated with the bar and its member stars have similar abundance and age. But several other streams found in the solar neighbourhood have been shown to also contain stars of different ages and metallicities, so they cannot be dissolving star clusters. They must have a dynamical origin probably associated with spiral structure. TMT/HROS observations for selected stars down to main sequences in these streams or moving groups are of high interest.
Meanwhile, the most outstanding result from chemical tagging of stars in the solar neighbourhood is that some stars with low-alpha ratios detected by Nissen & Schuster (1997, 2009) and they are thought to constitute the accretion component of the Galaxy from nearby satellite galaxies. In view of this, it is interesting to search for low-alpha stars in the region far outside the solar neighbourhood, especially in the outer halo, to investigate the merging history of our Galaxy. It is probable that the accreted subpopulations exist throughout the halo, considering the number of streams expected in ΛCDM is in the hundreds. These ‘missing satellites’ may have lose their space and kinematical record of the parent cloud, but may eventually be detectable through the chemical tagging. In addition, the outer halo formed from merging is considered to be younger than the dissipative component. Therefore, the abundance analysis and age dating of stars in the outer halo would provide crucial information on the merging history of the Galaxy.The best way to carry out this study is that TMT/WFOS will be used to obtain the spectra of a significant number of stars in outer halo, from which we can select interesting targets for HROS follow-up observations.
Finally, it would be interesting to compare the merging history of the Galaxy with those of nearby galaxies (see Sect. 4.6.1) in order to understand the theory of galaxy formation.

The abundances of stars in globular clusters and surrounding fields

Globular clusters (hereafter GCs) play an important role in modern astrophysics because they are the oldest objects in our Galaxy and some of them (e.g. young GCs in the outer halo) may originate from merging satellites.


Of our great interests, some of the key issues involving GCs are listed as follows. Firstly, we want to know to what extent there are multiple populations in GCs and to what extent are they chemically homogenous. The chemo-dynamical information of stars in the lower main sequences of GCs with TMT/WFOS will provide useful information on these issues. Secondly, it is commonly believed that the globular clusters seen in the Galaxy today are only a subset of those existed; some may have been shredded. It will be of interests to compare their chemical characteristics of GCs with the field population in order to probe whether the Milky Way halo could have been made at least in part by dissolving GCs and whether the GCs and the halo have the same metallicity distribution function, and whether they have the same chemical inventory. Thirdly, tidal tails of GCs are extremely interesting objects because they betray the existence of objects that were destroyed long ago and thus reveal the Galaxy’s accretion history. Presently, at least 30 Galactic GCs are likely surrounded by stellar tails as inferred from color-magnitude selected star counts (Grillmair et al. 1995; Leon et al. 2000). A spectroscopy survey of stars in the tidal radius and the detected tidal tails of GCs with WFOS/TMT is desirable. Finally, convincing measurements of [Fe/H] and key abundance ratios of dwarfs in GCs, which will require high S/N, high-resolution spectroscopy obtained with TMT/HROS, are important for tracing Galactic history (via the knowledge of early SN, star formation, and formation of the Galactic halo) and can be used as a way of understanding the nature and evolution of the diffusion history of elements during stellar evolution.
Possible TMT programs:

  1. It is interesting to trace the merging history of the Galaxy by deriving chemical compositions of stars in stellar streams, moving groups on the one hand and by searching for low-alpha stars (or other stars with anomalous abundances) from the solar neighborhood to the very outer halo of the Galaxy on the other hand. Some targets are selected from LAMOST low and middle resolution spectra. The combination of WFOS and HROS will be efficient for tracing the merging history of the Galaxy.

  2. Chemical abundances of stars in different stages in a few globular clusters in order to probe the diffusion history of elements and to investigate the characteristics of globular clusters themselves. The abundance analysis of stars in the tidal tails of GCs with WFOS for thousands of stars and HROS for limited sample of stars will be carried out.


China’s strengths and weakness in this area:

There are already several people working on abundance analysis of stars at NAOC and PKU. The stellar spectroscopic survey with LAMOST will provide interesting targets for TMT/HROS observations. Theoretically, we need to establish reliable chemical and dynamical evolution models to reproduce the observational results.


References:

  1. Grillmair C.J., Freeman K.C., Irwin M., Quinn P., 1995, AJ, 109, 2553

  2. Leon S., Meylan G., Combes F. 2000, A&A, 359, 907

  3. Nissen P.E. & Schuster W.J, 1997, A&A, 326, 751

  4. Nissen P.E. & Schuster W.J, 2009, astro-ph/0807.3831



4.6.4Isotope abundances


Jiangrong Shi, Gang Zhao (NAOC)
Key questions:

  1. When did AGB stars begin to contribute to the Galactic chemical inventory?

  2. Where are the n-capture elements produced?

  3. Is there an 6Li plateau in metal-poor stars?


Introduction:
Measurement of isotopic ratios provides a completely new window into nucleosynthesis, galactic chemical evolution, mixing within stars and stellar evolution. Such data, when available, significantly improves our understanding of nuclear processes in various astrophysical sites. And most isotope fractions, unlike elemental abundances, are very insensitive to the model atmosphere parameters, and the isotopic fractions can be measured by detailed comparisons of an observed absorption line profile to synthetic spectra of these line substructures. However, because the isotopic shift is generally very small at optical wavelengths, very high spectral resolution (at least 90,000) is required to measure isotopic ratios in stellar spectra.
Main science areas:
1. The isotopic ratios of Mg in metal-poor stars with TMT/HROS
Magnesium (Mg) has three stable isotopes with atomic weights 24, 25 and 26, the lightest isotope 24Mg is produced via burning of carbon in the interior of massive stars during their normal evolution as a primary isotope, while the two heavier isotopes 25, 26Mg are secondary isotopes produced primarily in intermediate mass AGB stars. Thus, the isotopic ratios 25, 26Mg/24Mg increase with the onset of AGB stars. Therefore, Mg isotopic ratios in halo stars could be used to constrain the rise of AGB stars in our Galaxy (Meléndzland & Cohen 2009).
It is important to know when AGB stars begin to enrich the halo in order to disentangle the contribution of elements produced by intermediate-mass stars from the contribution of elements. And the Mg isotopic abundances can be derived based on the analysis of MgH lines in cool metal-poor stars.
2. The isotopic ratios of n-capture elements with TMT/HROS
The elements heavier than the iron peak are mainly produced through neutron capture reactions in two main processes, the s-process (slow) and r-process (rapid). Unlike other elemental abundances, any n-capture element with multiple naturally occurring isotopes that are produced in different amounts by the s- and r-processes can be used to assess the relative s- and r-process contributions to the stellar composition. So, the isotopic abundances for these elements are more fundamental indicators of n-capture nucleosynthesis, because they can be directly compared to r-process and s-process predictions without the smearing effect of multiple isotopes (Mashonkina et al. 2003, 2006).
As supported by many observational and theoretical results, s-nuclei are mainly synthesized during the thermally pulsing asymptotic giant branch phase of low-mass stars. The r-process is associated with explosive conditions in SNe II, however, the precise astrophysical sites of the r-process have not been identified. To reconstruct the evolutionary history of neutron-rich elements in the Galaxy it is thus important to demonstrate that the r-process elemental abundance pattern extends to the isotopic level, which would greatly strengthen the argument for a universal r-process mechanism for the heavy n-capture elements.
The combination of Ba, Nd, Sm and Eu isotopic fractions can provide more complete knowledge of the n-capture nucleosynthesis, constrain the conditions (e.g., temperature, neutron density, etc.) that are required to produce the r-process elements, and determine the actual r-process path by identifying the individual isotopes that participate in this process (Roederer et al. 2008).
3. The isotopic ratios of Li in metal-poor stars with TMT/HROS
Lithium has two stable isotopes, 6Li and 7Li. For the Big Bang nucleosynthesis theory, it is important to determine the 6Li/7Li ratio in extremely metal poor dwarfs. Lithium is destroyed by nuclear reactions at high temperatures. In the presence of mixing, material from the stellar atmospheres can reach depths of high temperatures where lithium can burn or mix with material that has burnt it. According to some stellar evolution models, little or no 7Li destruction is expected for stars in the Spite plateau, so that these stars should retain their initial content. 6Li is, however, more fragile than 7Li, so an observable content of the lighter isotope sets constraints on the degree of depletion of the other one.
These are two challenging questions concerning the abundances of 6Li and 7Li in very metal-poor stars: First, predictions of the nucleosynthesis by the big bang theory are tightly constrained because the anisotropies of the cosmic microwave background determine the only free parameter in the standard cosmological model. The predicted abundance of 7Li is about 0.5dex larger than the measured abundance of lithium on the Spite plateau. So, the question is how does one bridge the 0.5 dex gap between observation and prediction? Second, the results from Asplund et al. (2006) suggested that there may be a 6Li plateau parallel to the Spite plateau for 7Li, which implies a major fraction of the 6Li may have been synthesized prior to the onset of star formation in our Galaxy. The question is thus: how do we account for the high abundance of 6Li in some metal-poor stars? Confusingly, the new results from Steffen et al. (2009) do not support the 6Li plateau. So it is important to determine the 6Li/7Li ratios for more metal-poor stars with TMT/HROS to answer these two questions.
While, the 12C/13C ratio is a sensitive indicator of the mixing processes experienced by carbon-enhanced stars. As its ratio largely unaffected by uncertainties in the adopted stellar parameters, and appears to be high in nearly primordial gas. Thus, any significant variation of 12C/13C should be due to internal mixing processes in the stars. Thus, the measurements of this ratio are a particularly powerful tool to constrain the models of internal mixing in giant stars, especially in extremely metal-poor giants.
Possible TMT programs:
1) Determine isotopic ratios of Mg and Li in selected metal-poor stars.

2) Determine isotopic ratios of n-capture elements in a sample of halo and disk stars.



3) Determine iotopic ratios of C in metal-poor and Globular cluster giant stars.

China's strengths and weakness in this area:
In China, several teams, especially at NAOC and PKU, have experiences on analyzing the chemical abundances of stars in our Galaxy, both for the disk and halo stars. Their works are well known internationally in the field, and they have been involved in many international collaborations. They have analyzed high-resolution echelle spectra of many stars observed with many telescopes 2m (NAOC; Germany/Calar Alto), 6m (in Russia) and 8m (Sabura, VLT) optical telescopes. The isotope abundance ratios for some important elements are determined taking into account non-local thermodynamic equilibrium effects. Recently, these teams are working on LAMOST surveys, which will provide huge database of spectra of Galactic stars. Together with follow-up high-resolution observations, we expect to greatly increase the total number of (extremely) metal-poor stars, thus making significant contributions to the field.
However, we have only limited opportunity to directly obtain high-resolution spectra with 8-10m telescopes. TMT should remedy this situation. Combined with our theoretical expertise, we will be able to make new, internationally competitive contributions in this area.
References:

  1. Asplund, M., Lambert, D. L., Nissen, P. E., Primas, F., Smith, V. V., 2006, ApJ, 644, 229

  2. Mashonkina, L., Gehren, T., Travaglio, C., Borkova, T., 2003, A&A, 397, 275

  3. Mashonkina, L., Zhao, G., 2006, A&A, 456, 313

  4. Meléndzland, J., & Cohen J. G., 2009,ApJ, 699, 2017

  5. Roederer, I. U., Lawler, J. E., Sneden, C. et al. A., 2008, ApJ, 675, 723

  6. Spite1, M., Cayrel1, R., Hill1, V., et al. 2006, A&A, 455, 291

  7. Steffen, M., Cayrel R., Bonifacio, P., Ludwig, H.-G., Caffau, E., 2009, arXiv0910.5917



4.6.5Chemical abundances of resolved giant stars and HII regions in Local Group dwarf galaxies


Yanchun Liang (NAOC), Yuqin Chen (NAOC), Gang Zhao (NAOC), Jianrong Shi (NAOC)
Key questions:

  1. How do we understand the evolutionary and chemical enrichment histories of local group dwarf galaxies?

  2. What is the relationship between dwarf Irregulars (dIrr) and dwarf spheroidals/dwarf ellipticals (dSphs/dEs)?

  3. What are the [α/Fe] abundance ratios of stars in the dSphs?

  4. How do the HII regions abundances correlate with the luminosities for dIrr galaxies?


Introduction:
The Local Group is a collection of dwarf galaxies dominated by two giant spirals, our Milky Way (MW) and Andromeda (M31). The dwarfs of the Local Group provide a uniquely well-studied and statistically useful sample of low-luminosity galaxies. This includes a number of different types: early-type dwarf spheroidals/dwarf elliptical (dSphs/dE), late-type star-forming dwarf irregulars (dIrrs), the recently discovered very-low surface brightness, ultrafaint dwarfs (uFds), and centrally concentrated a actively star-forming blue compact dwarfs (BCDs). Tolstoy, Hill, Tosi (2009) [1] and Mateo (1998) [2] have nicely reviewed the progress on dwarf galaxies in the Local Group (see Appendix B). With ground-based 8-m telescopes, it is possible to obtain detailed chemical abundances of the resolved red giant branch (RGB) stars in dSphs; the abundances of HII regions in dIrrs can be obtained by even smaller telescopes since they are brighter. Great progresses have been obtained for these subjects, but still lots of uncertainties exist. With the 30m TMT and efficient instruments, such as HROS and WFOS, we will significantly improve our ability to study resolved stellar populations in Local Group galaxies. Moreover, TMT has its unique opportunity to observe the northern sky (from Hawaii) since GMT and E-ELT will be both located in Chile (in the Southern Hemisphere).
Main science areas:
1. The chemical abundances of resolved RGBs in dSph galaxies using TMT/HROS

With the high resolution of TMT/HROS, it is possible to obtain the Echelle spectra of the resolved RGB stars in the dSphs in Local Group in the northern sky. These can trace the stellar population and star- formation history of the galaxies and study the statistic properties of this kind of galaxies.


More than one hundred RGB stars in the local dSphs galaxies mostly in southern sky have been measured their chemical abundances, especially the [α/Fe], by using the 8-10m telescopes. The first studies on detailed chemical abundances in dSphs galaxies are using Keck-HIRES (Shetrone et al. 2001[3], 17 stars in Draco, Ursa Min, Sextans) and VLT-UVES (Bonifacio et al. 2000 [4], 2 stars in Sgr). These early works were shortly followed by similar studies slowly increasing in size. Most recently, the DART survey determined kinematics and metallicities for large sample (>80) individual stars in nearby dSphs by using the VLT/FLAMES multifiber facility with high spectral resolution. There have been similar surveys by other teams on VLT and Magellan telescopes. All these are for Sgr, Fnx, Carina, Sci etc. Aoki et al. (2009) [5] has used the Subaru/HDS to observe 6 metal-poor stars in Sextans. These stars typically have magnitudes in the range V=17-19, and the exposure time is up to 14 hours even for 8-10m telescopes.
With the great 30-m TMT, and the high resolution spectrograph HROS, R~30000- 50000, we could observe a number of resolved RGBs in dSphs in northern sky. There could be 28 such dSph within distance of 2200 kpc [1, 2]. HORS will be able to obtain similar results for stars at V~20 in more distant (~400-500 kpc) and hence more isolated systems, even can try for farther systems. The optical measurements can also be supported by K-band measurement: TMT/NIRES can easily reach below the tip of the red giant branch throughout the Local Group. These much extensive studies not only allow us to obtain the abundance pattern in individual galaxy, but also are important for statistical studies. The color-magnitude diagrams (CMDs) of nearby dwarf galaxies can be used to find the targeted stars. Since 1950s, large numbers of detailed CMDs have been derived for star clusters and nearby dwarf galaxies. To date, for many of the galaxies HST/WFPC2 data already exist [1].
The α-elements abundances are good tracers for the chemical evolution histories of galaxies. They can easily be measured in RGB spectra includes O, Mg, Si, Ca, Ti. Fig.11 of reference [1] shows the Mg and Ca abundances of individual star in those dSphs with more than 15 measurements. It shows that each of these dSphs starts at low metallicity, where its [α/Fe] ratios are similar to those in the MW halo, and then evolve down to lower values than are seen in the MW at the same metallicity. There is a "knee" in a plot of [Fe/H] versus [α/Fe]. The knee position indicates the metal-enrichment achieved by a system at the time SNe Ia start to contribute to the chemical evolution. The knee is not well defined in dSphs owing to a lack of data except in Scl dSph. With HROS, we could study the [α/Fe] abundances of the resolved RGBs, and the "knee" in the individual dSph in northern sky. We could further check whether the position of the knee correlates with the total luminosity and/or the mean metallicity of the galaxy.
Moreover, [Na/Fe], [Ni/Fe], [s-process elements/Fe] abundances can also been studied to better understand the chemical enrichment history of these dSphs. We could also compare the abundance patterns between the dSphs around M31 and MW, and check if there exists any difference between them, which could give hints for the environments and formation histories of M31 and MW. Detailed chemical abundances of individual stars in M31, M33, M32 could also been studied and compared with those in MW.

2. Chemical abundances of resolved stars in uFds with TMT/HROS

A number of Ultrafaint Dwarf galaxies (uFds) are being found by the SDSS around the MW[1]. Some of these new systems have had their stellar population analyzed using the synthetic CMD method. The individual stars in the uFds have so far been little observed at high-resolution. This is probably owing to the difficulty in confirming membership for the brighter stars in these systems. Several groups (Koch et al. 2008[6], Frebel et al. 2009[7]) are currently following up confirmed members (typically selected from low resolution Ca II triplet observations) to derive abundances. With the TMT/HORS, we could observe the individual RGB stars in these uFds and to study their chemical abundances. All the 19 uFds are dSphs (see http://www.delphes.net/messier/more/local.html). The HROS observations will be good follow-up for the middle resolution observation with WFOS (see Section 4.6.1).


3. Dwarf Irregular galaxies: abundances of HII regions with WFOS and abundances of resolved supergiants with HROS
The dIrrs are all (except the SMC) located at rather large distances from the MW. So far, the only probes that could be used to derive chemical abundances in these objects are HII regions and a few supergiant stars. This limitation makes it difficult to gather relevant information to constrain the chemical enrichment histories of these systems. However, abundances in HII regions and supergiants are useful to understand how dIrrs fit in the general picture of dwarf galaxies, and how they compare to larger late-type galaxies.
The well quoted work of Skillman et al. (1989) [8] provided the luminosity-metallicity relations for a sample of local dIrr galaxies within ranges of 12+log(O/H) from 7.3 to 8.3 and MB from -10 to -19 mag. Candidate HII regions in the 7 nearby dwarf irregular sample galaxies were selected from an Hα survey of dwarf irregular galaxies. Hodge et al. and Hunter et al. have made lots of efforts on surveys on Hα in nearby irregular galaxies.
With TMT/WFOS (R<5000), it is possible to obtain the optical spectra of a number of HII regions of dIrr galaxies in the Local Group in northern sky and study their HII region abundances. The luminosity-metallicity relation could be checked further in the metal-poor and low-luminosity ranges. This will extend the present SDSS luminosity-metallicity relation and stellar mass-metallicity relations found for a large sample of metal-rich star-forming galaxies.
These local dIrrs will be good places to estimate the primordial He abundance. Their low abundances of metals and helium, derived from HII region spectra in the dIrrs, allow the determination of the primordial helium abundance with minimum extrapolation, and thus provide insight into Big Bang nucleosynthesis.
A- to M-type supergiants are of further interests as they provide the present-day [α/Fe] ratios in dIrrs. The first dIrr where abundances of stars were measured was the SMC. Similar studies in more distant dIrrs needed efficient spectrographs on 8-10m telescopes at the expense of observing for many hours. Venn et al. (2003)[9] study the A-type stars in NGC6822, and Tautvaisiene et al. (2007) [10] study the M-type stars in IC 1613. This small sample shows that the dIrrs actually extends the trends of dSphs. TMT/HROS will dramatically improve observations on individual stars in the dIrrs and allow us to further compare the abundance patterns of dIrrs and dSphs.
Possible TMT programs:


  1. A survey to study chemical abundances of the resolved RGBs in dSphs.

  2. A survey to study chemical abundances of the RGBs in uFds.

  3. Surveys for chemical abundances of HII regions and supergiants in dwarf Irregular galaxies, the primordial He abundance.



China's strengths and weakness in this area:
The science case mentioned in Sect. 4.6.1 about the resolved stellar populations and chemical abundances in nearby dwarf galaxies are of fundamental importance in astronomy. Several teams, especially at NAOC and PKU, have great experience on analyzing the chemical abundances of stars in our MW, both in the disk and halo, and of the HII regions and Planetary Nebulae (PNe). Their works are well known internationally, and they are involved in many high-profile international collaborations. They have observed and analyzed the high-resolution Echelle spectra of many stars and PNe using 2m (e.g., the 2.16m of NAOC), 6m (in Russia) and 8m (Subaru) optical telescopes. For low-resolution spectra, a large sample of HII regions (and PNe) and star-forming galaxies, especially from SDSS, have been analyzed to study the metallicities and stellar populations. All these experiences will be useful for analyzing the observed spectra of the resolved stars and HII regions in nearby dwarf galaxies. Also these teams are working on LAMOST surveys, which will provide huge database of spectra of Galactic stars and extragalactic objects. These can also be related to the chemical Evolution models of galaxies done at NAOC, SHAO, and SNU.
However, due to the lack of observational facilities, we have no opportunity to directly observe and analyze the chemical abundances of resolved stars in Local Group galaxies with 8-10m telescopes. TMT will allow the Chinese astronomers to compete globally in this area using first-hand data by forming a coherent, stable research group including young astronomers and students.
References :

  1. Tolstoy, E., Hill,V., Tosi, M. 2009, ARA&A, 47: 371-425

  2. Mateo, M. 1998, ARA&A, 36: 435-506

  3. Shetrone, M. D., Côté, P., Sargent, W. L. W. 2001, ApJ, 548 : 592-608

  4. Bonifacio, P., Hill, V., Molaro, P. et al. 2000, A&A, 359: 663-668

  5. Aoki, W., Arimoto, N., Sadakane, K. et al. 2009, A&A, 502: 569-578

  6. Koch, A., McWilliam, A., Grebel, E.K., Zucker, D.B., Belokurov, V., 2008, ApJ, 688: L13-L16

  7. Frebel, A., Simon, J.D., Geha, M., Willman, B. 2009, arXiv0902.2395

  8. Skillman, E. D., Kennicutt, R. C., Hodge, P. W., 1989, ApJ, 347: 875-882

  9. Venn, K. A., Tolstoy, E., Kaufer, A. et al. 2003, AJ, 126: 1326-1345

  10. Tautvaišien, G., Geisler, D., Wallerstein, G. et al. 2007, AJ, 134 : 2318-2327


4.6.6Mass distributions of the Milky Way and local group


XiangXiang Xue (NAOC)Gang Zhao (NAOC)
Key questions:


  1. What is the total mass and dark matter distribution of our Milky Way? Are all the Milky Way's satellites bound?

  2. How does the mass distribute in the nearby galaxies?


Introduction:
The mass distributions of the Milky Way and the nearby galaxies are unclear as a result of limited capability of current telescopes. Understanding the mass distributions of the Milky Way and nearby galaxies is crucial for modeling the dynamics of the local group, connecting observations of galaxies to large-scale cosmological dark matter simulations. Mapping the mass of the Milky Way and the nearby galaxies demands good tracers with precise positions and velocities. Furthermore these tracers should distribute in the galaxy as far from the center as possible. The standard candle, such as blue horizontal branch stars (BHB, Mv=0 [1]), red clumps giant (RCG, 0.5v<1.3 [2]), super giants (SG, -9v<-4 [3]), Cepheid variables (CV, -6v<-2 [4]) and RR Lyrae variables (Mv=0.61 [5]) are very luminous and their distances can be determined easily and accurately. TMT will provide a chance to obtain such tracers in the distant Galactic halo and nearby galaxies.
Main Science areas:
1. Dark matter halo mass of the Milky Way

Our internal position permits the placement of unique constraints on the Galaxy’s stellar mass content, its dark matter profile at large radii, and the three-dimensional shape of its dark matter halo. Yet our location within the Galaxy also complicates some measurements, such as the extended rotation curve of gas in its disk. As a result, the dark mass profile for the Milky Way beyond 20 kpc and the total mass of the Milky Way have not been previously constrained to better than a factor of 2-3. The latest estimate of the Milky Way’s total mass was derived by the kinematics of 2400 BHB stars drawn from SDSS DR6 [6], although these stars distributed only up to 60 kpc from the Galactic center. SDSS DR8 provides about 4600 BHB stars up to 80 kpc from the Galactic center [7]. The BHB sample has been enlarged two times, but many faint BHB stars are lost because of bad quality spectra. TMT provide us a great opportunity to obtain high quality spectra of distant halo stars. TMT observations of extreme distant halo BHB stars and RR Lyraes may provide unique constraints on the Milky Way mass.


2. Mass distribution of nearby galaxies

In recent years many studies have been done to explore nearby galaxies, such as M31, M33, etc [8,9,10,11,12], but it is hard to go further into M31 and other fainter nearby galaxies due to the limitations of current telescopes. TMT enables spectroscopic observations of standard candle stars (BHB, RCG, SG, RR Lyrae, CV) in the nearby galaxies even fainter ones (see also Sect. 4.6.1). The distances and velocities of these stars can help us to explore the mass distributions of the galaxies, and check the cosmological simulations of the galaxies.


Possible TMT programs:


  1. A spectroscopic observation of faint BHB stars and RR Lyraes in the Milky Way with WFOS; the faint BHB stars and RR Lyraes candidates may be obtained by SDSS/SEGUE and Pan-STARRS.

  2. The spectroscopic observations of standard candle stars of nearby galaxies with WFOS. The radial velocities and precise 3D positions of these special stars enable mapping the mass distributions in the galaxies.


China’s strengths and weakness in this area:

We have the experience on the research of mass distribution of the Milky Way. There are already several people working on cosmological galaxy formation simulations. We are internationally competitive for connecting observations to the theoretical simulations.


We still need to do more efforts on the simulations of nearby galaxies.
References:

[1] Yanny, B. et al. 2000, APJ, 540, 825

[2] Groenewegen , M. A. T. et al. 2008, A&A, 488, 935

[3] Slowik, D. J. et al. 1995, AJ, 109, 5

[4] Chiosi, C. et al. 1993, APJS, 86, 541

[5] Benedict, G. F. et al. 2002, AJ, 123, 473

[6] Xue, X.-X et al. 2008, APJ, 684, 1143

[7] Xue, X.-X et al. 2010, in prep.

[8] SarajediniAta, Yang, S. -C. 2009, arXiv0912.2863S

[9] Yang, S.-C.; Sarajedini, Ata, 2010, ApJ, 708, 293Y

[10] Fiorentino, G. et al. 2009, arXiv0911.0355F

[11] Forestell, A. D. et al. 2004, ASPC, 310, 99F



[12] Massey, P. et al. 2006, AJ, 131, 2478M

4.6.7Spectroscopic study of low-mass stars, brown dwarfs and planets


L.F. Li, Z.W. Han, S. H. Gu, F.H. Zhang, X.F. Chen, R. S. Pokorny, J.H. He (NAOC/YNAO)
Key question:

  1. How the stellar evolution theory is calibrated?

  2. What is the physics that controls the production of magnetic fields of very low-mass fully-convective stars and brown dwarfs?

  3. What is the properties of brown dwarfs

  4. How does a host star interact with its planet through the magnetic field?


Introduction:
This project would like to obtain the spectra of some low-mass, cool stars (including those with a convective envelope and fully convective ones) or even brown dwarfs with high resolution and high signal-to-noise (S/N) using TMT.
Main science areas:
1.The calibration of stellar evolution theory
Stellar evolution theory is sometimes considered to be a mature branch of astrophysics with well-established results and only minor need for further research. This is a misconception. The quality of a physical theory is always to be measured against experimental (observational) facts [1]. We do not understand the stars well enough. There are many physical processes which are either not well understood, or not taken into account properly in the theoretical models of stars, such as convection, rotation, diffusion, atmospheres, and mass loss, etc.
Convection is a very important means for the transport of both energy and matter. It is intrinsically time-dependent, non-local, and 3-dimensional. It is a long-standing problem of stellar evolution theory that it cannot be treated adequately in the calculations. The standard method still assumes that the mixing due to convective flows is instantaneous and computes the resulting temperature gradient in super-adiabatic layers according to a simple Mixing-length theory in which only one length scale for transport of convection elements is assumed. Although the theory of atmospheres is treated usually independently from that of the interiors of stars, the comparison of the models with observations inevitably involves it. In particular the cool stars are difficult, because of the formation of molecules and dust grains and the interaction with convection, pulsation, stellar winds. Atmospheres should also provide mass loss rates, but so far only for hot luminous objects the theory is far advanced [1]. Stars are known to rotate and some do so rapidly. The effects on the stellar structure and evolution of are two-fold: the spherical symmetry is broken (the hydrostatic equilibrium is modified due to the rotational forces), and large- and small-scale matter motions might be induced, leading to rotationally induced mixing.
We would like to use TMT to observe the spectra with high quality for some low-mass single stars with different spectral types (G and K) or detached binaries contained these cool stars. Then we can derive the mass loss rate and improve atmosphere model for these cool stars through the spectroscopic analysis. These results will be used to limit the treatment of convection and rotation effects, since the atmosphere always interacts with the convective envelope. These results will in turn be used in stellar evolution models to remove some uncertainties and improve the determination of stellar age scales. These absolute and relative age-scales of the stars can offer an opportunity to compare independently with cosmological models and parameters obtained from other source (e.g. cosmic microwave background and SN Ia). Meanwhile, we analyse the spectra of some detached binaries that contain cool components and derive their radial velocities and the accurate physical parameters (masses, radii, luminosities and effective temperatures). By a comparison between these observed results and the theoretical models we can calibrate stellar evolution theory. Then, we use these results that have been calibrated to investigate the evolution of the distant globular clusters and galaxies based on population synthesis.
2.The spectroscopic study of fully-convective stars and brown dwarfs
Very low-mass stars with late spectral types (M, L) are the majority constituent of the Galaxy by number. Their lifetimes are much greater than the current age of the universe, and they therefore serve as useful probe of Galactic star formation history in the local solar neighborhood [2]. They also encompass many important regions of stellar parameter space, including the onset of convection in the stellar interior, the onset of significant electron degeneracy in the core, and the formation of dust and subsequent depletion of metals onto dust grains in the stellar atmosphere. Of particular interest is the fact that many late type stars have strong surface magnetic fields [3] that heat the outer atmosphere above the photosphere and lead to observable emissions from the chromosphere (e.g. Ca II and H Balmer series), the transition region (e.g. resonance lines of abundant ions such as CIV) and corona (e.g. thermal soft X-rays). The very low-mass fully-convective stars (M3 to ~M6) and brown dwarfs (Later than ~M6) is very different from the solar-like stars with a convective envelope. The solar dynamo theory is proposed on base of the structure of the Sun which has a radiative core and a convective envelope. It suggests that the magnetic fields of the solar-like stars usually origins on the base of convective zone, then it is enlarged by differential rotation. Therefore, the physics of that controls the production of magnetic fields in very low-mass fully-convective stars and brown dwarfs is not well understood [4]. Meanwhile, the rotation evolution of very low-mass stars and brown dwarfs shows that the normalized magnetic activity level does not weaken until spectral type mid-M (M 6-7), after that its diminishes rapidly [5]. The physics that underlie the weakening of magnetic activity of stars with spectral types later than M6-7 is not clear. Mohanty & Basri [6] suggested that the atmospheres of the stars with spectral types later than M6-7 might become neutral, however it requires the supports of observations.
We want to observe the spectra for some very low-mas stars and brown dwarfs with different spectral types (from M to L) by using TMT, such as Gl 70 (M2.0, J=7.37mag), Gl 729 (M3.5, J=6.22mag), Gl 876 (M4.0, J=5.93mag), Gl 299 (M4.5, J=8.42mag), Gl 905 (M5.0, J=6.88mag), GJ 1002 (M5.5, J=8.32mag), GJ 1111 (M6.0, J=8.24mag), LHS 3003 (M7.0, J=9.97mag), VB 10 (M8.0, J=9.91mag), LHS 2924 (M9.0, J=11.99mag), LHS 2065 (M9.0, J=11.21mag) and so on. By analysis of these spectroscopic observations, we would investigate the atmospheres (including the chemical elements and property of the atmospheres), the physics that controls the production of magnetic fields and rotation in these objects. For example, the effective temperature of these very cool dwarfs is very low and the atmosphere might be neutral. If so, no ionised components can be found in the atmospheres of the stars with spectral type later than M6-7. The neutral atmosphere would weaken the efficiency of magnetic braking, since the neutral matter is no longer lost at the Alfven radius as the ionised gas of hot stars. The mass of dwarfs (~13-75MJup) locates a region between the planets and low-mass stars, the systemic spectroscopic study in the brown dwarfs is important not only for investigating the evolution of brown dwarfs, but also for investigating the formation of planets (see below).

3. The magnetic interaction between the host star and its planet
For the hot Jupiters lied within 0.1AU to the host star, normally they are located within the Alfven radius of the host stars. Such a condition permits the magnetic interaction between the host star and its planet [7]. The study for such phenomenon can help us to understand the formation, migration and evolution of hot Jupitors, and also provides a way to detect the magnetic field of the planet, offering us a chance to explore the interior structure of extrasolar planet and understand the hydrodynamic property of extrasolar planet's atmosphere.
Normally, such an interaction is weak, we can find the interaction signal from the variation of core emission of CaIIHK lines, or using Doppler imaging to detect the magnetic variation in time series of spectral profiles. Such observations will require large telescope like TMT/HROS.
Possible TMT programs:


  1. Obtain high S/N spectra for low-mass stars with different spectral types, and then derive the mass loss rate and improve atmosphere model for the cool stars through spectroscopic analyses.

  2. Systematic spectroscopic study of a sample of brown dwarfs.

  3. Select a group of stars with close-in planets, monitor the CaIIHK lines and detect the variation of their core emission, and perform Doppler imaging of extrasolar planet systems to find the change of magnetic field of the host stars due to the interaction.

Strengths and weakness of Chinese astronomy in this topic:

There are two groups working in this field at YNAO. One group mainly investigate the structure and evolution of the single and binary stars by constructing their evolutionary models theoretically, then, investigate the evolution of the globular clusters and galaxies by using population synthesis based on stellar evolution theory. Therefore, the uncertainties in stellar evolution theory would have a significant influence on the investigation of globular clusters and galaxies. However, we cannot remove the uncertainties in the stellar evolution theory because of the lack of the support of observational results with a high quality for cool stars and brown dwarfs. Another group working in stellar magnetic fields, and they have already established a platform for such a research topic. They need to further train PhD students, particularly on the magnetic interaction between host star and planet so that we can perform research in this area more efficiently.


Reference:



  1. Weiss A., 2002, ESASP, 485, 57

  2. Gizis J.E., Reid I.N., Hawley S.L., 2002, AJ, 123, 3356

  3. Johns-Krull C.M., Valenti J.A., 1996, ApJ, 459, L95.

  4. West A.A., Hawley S.L., Walkowicz L. M., Covey K.R., et al., 2004, AJ, 128, 426

  5. Schmidt S.J., Cruz K.L., Bongiorno B.J., Liebert J., Reid I.N., 2007, AJ, 133, 2258

  6. Mohanty S., Basri G., 2003, ApJ, 583, 451

  7. Shkolnik, E. Bohlender D.A., Walker G. A. H. et al. ApJ, 2008, 676, 628

4.6.8Observational Studies of Neutron Star Systems


Zhongxiang Wang (SHAO)
Key questions:

1. Do fallback disks generally exist?

2. What is the optical/near-infrared radiation mechanism in magnetars?
Introduction:

While since their discovery in 1967, neutron stars (NSs) have been primarily targets at radio frequencies, observational studies have been extended to optical, X-ray, Gamma-ray, and more recently infrared (IR) frequencies because of various manifestations of them over the frequencies. With TMT, much improvement on our understanding of certain aspects of them can be made.


Searching for fallback disks

Fallback occurs in core-collapse supernovae when the reverse shock, caused by the impact of the shock wave with the outer stellar envelope, reaches the newly formed NS. Fallback could have profound implications for the endgame of massive star evolution, particularly since it could lead a newborn NS to collapse into a black hole. It is also a promising mechanism for producing the debris disks necessary to form planets, which are known to exist around at least one pulsar. It was once thought that young radio pulsars like those in the Crab and Vela supernova remnants were prototypical of newborn NSs, but recently it has been realized that the young NS population is unexpectedly diverse. For example, there are central compact objects (CCOs) that are mysterious radio-quiet, non-plerionic X-ray point sources, and anomalous X-ray pulsars (AXPs) and spectacular soft gamma-ray repeaters that are believed to have extremely strong (10^{14} G) surface magnetic fields (“magnetars''). This diversity requires explanation, and the existence of fallback disks could be part of the cause [see 1 and references therein].


Recently, Magellan near-IR and Spitzer mid-IR searches for fallback disks were carried out. The preliminary targets were several AXPs and CCOs, and a few radio pulsars. One candidate fallback disk was found around the AXP 4U 0142+61 [2]. The upper limits on other targets were approximately 5 micro-Jy at K band, Spitzer 4.5 and 8.0 microns. Considering these results, the searches are not conclusive because of the uncertainties on the source distance, interstellar extinction, and (putative) disks' geometry and orientation.
With TMT, 100 times better sensitivity at near-IR wavelengths will be reached for point sources such as neutron stars. By doing a similar search to that recently conducted and combining it with JWST observations at mid-IR, the question whether or not fallback disks generally exist around isolated NSs would be answered. The candidate fallback disk found around 4U 0142+61 needs further confirmation. With TMT, IR spectroscopy searching for lines in its emission would provide the answer.
In addition, as part of the targets, AXPs are found to in general have relatively bright optical and

near-IR emission (R>25, K>20), which is not seen in normal pulsars. Because of large distances and high extinction to them, this part of emission is not well observed and not understood at all. TMT will allow a detailed study of them, helping our understanding of the emission mechanism.


Pulsars and Pulsar Wind Nebulae

Radio pulsars are born spinning rapidly, with a large amount of rotational energy to expend. The major part of the rotational energy loss of a pulsar will be carried out in a magnetized, high-velocity stellar wind, which fuels an extended pulsar wind nebula (PWN) by shocking the surrounding slow-moving medium. Studies of PWNe are important for understanding astrophysical processes such as magnetized relativistic flows, shock interactions, and high-energy radiation mechanisms. Observations of them also offer ways of probing the interstellar medium [3]. Thanks to Chandra, ~40 PWNe have been detected and studied at X-ray energies. They appear as various types of shapes, having fine structures that indicate distributions of the magnetic fields and high-energy particles. Their spectra are power-law, arising from synchrotron radiation [4].


It has been considered that synchrotron spectra of PWNe rise from X-rays all the way to radio frequencies, with a break somewhere between the radio and IR frequencies. Such a spectral break

provides information about the age and magnetic field strength of a PWN. However, recently it has

been realized through IR observations of PWNe that multiple spectral breaks may exist, indicating the evolution of a PWN's magnetic field and the spectrum of particles ejected from the associated pulsar [5]. In order to better study PWNe by obtaining their broadband spectra, a TMT survey to cover optical and near-IR wavelengths would help. Previous targets that have been accessible to study with current telescopes are only a handful of brightest pulsars.
With such a survey, it would be likely to detect pulsars as well. While it is generally accepted that

pulsars have a power-law optical/IR spectrum, arising from synchrotron radiation of particles in the

magnetosphere, only 9 of ~1000 pulsars have been detected and most of the detections had low

signal-to-noise ratios (S/N) [6]. It is important to firmly confirm the previous results and establish this fact by detecting more pulsars. For a few middle-aged pulsars, a better determination of their power-law emission will improve studies of thermal emission from the surface of the NSs, which helps determine the thermal evolution of NSs and the equation of state of their super-dense interiors. In addition, the spectral turn-over point due to synchrotron self-absorption has been suggested to be within near-IR wavelengths [7]. Detections of the turnover points would probe the magnetospheric structure of pulsars.


Observations of Low-Mass X-Ray Binaries

a. Masses of accreting neutron stars

One effort in observational studies of low-mass X-ray binaries (LMXBs) is to measure the masses of NSs in them through time-resolved spectroscopy. NSs in LMXBs are accreting from their companions, and thus may have mass values larger than the canonical 1.4 Msun value. The ultimate goal is by collecting mass measurements of NSs, the long-sought equation of state of their superdense interiors can be determined. For certain cases, it has shown that the current 6-10 meter telescopes are still not sufficiently large, with low S/N detections of lines [e.g., 8]. A large collecting area of TMT will greatly help in this area.


b. Properties of ultracompact LMXBs

Ultracompact LMXBs are those binaries with orbital periods lower than 80 min. Along with their white dwarf analogues (the AM CVn binaries), they represent extreme and exotic endpoints in binary and stellar evolution. For the persistent ultracompact systems, their optical emission is dominated by that from their accretion disks. It is believed that the companions in ultracompact binaries must be hydrogen-poor. In several sources, based on the orbital periods, the companions are suggested to be C/O white dwarfs [9]. Spectroscopy of them with TMT will help find direct evidence for the suggestion; they are too faint to the current large telescopes. In addition, there are a few candidate ultracompact LMXBs, selected based on either their optical brightnesses or X-ray spectral features. Time-resolved photometry with TMT will help determine whether they are ultracompact and have orbital periods of ~10 min. The limiting orbital period that can be detected by the current large telescopes is approximately >15 min for these sources (V=21).


The known transient systems, which outburst once in every few years, also contain a millisecond X-ray pulsar (MXP). The discovery of the first MXP binary in 1998 first and finally confirmed the long-sought connection between LMXBs and recycled millisecond radio pulsars [10]. Thus far, the transient systems have only been observed in their outburst, with optical emission coming from an X-ray heated accretion disk. What the companion stars are may be guessed from the obtained ultra-short orbital periods. Observations of the sources in their quiescence are required in order to learn the detailed properties of them. Recent such effort with Very Large Telescopes failed [e.g. 11], indicating the need of TMT.
Possible TMT programs:

  1. Searching for fallback disks around a sample of isolated, young neutron stars.

  2. Optical/near-IR observations of pulsars and their wind nebulae.

  3. Spectroscopy of low-mass X-ray binary systems; Ultracompact X-ray binary identification.


China's strength and weakness in these sub-fields:

There are a few people working on observational and theoretical studies of the sub-fields. We are internationally competitive, although much effort is needed to build up world-leading groups.


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