This is a program to identify white dwarf (WD) stars in nearby star clusters with turnoff mass between 4 and 10 solar masses. The goals are to determine the critical main sequence mass above which stars explode as core-collapse supernovae and below which they evolve into WDs and to map the initial-mass final-mass relation for stars that become WDs.
In the first years of the program we were active collecting the basic observational material for the program. We have been very successful in being awarded telescope time for this project. To date, we have been awarded eight nights with the Lick Observatory 3m telescope Prime Focus Camera, two nights with wide-field MegaCam at the MMT, three nights with the CTIO 0.9m imager and four nights with the CTIO 4m Mosaic imager all for the deep, wide-field photometric surveys required to identify candidates. An additional five nights have been award at the Keck 1 10m telescope with the low-resolution imaging spectrometer to obtain the crucial followup spectra.
A second major activity has been to improve, document and make more robust the analysis code used to accurately determine, T(eff), log(g), luminosity and mass of each WD discovered in our survey.
Activities July 2005--June 2007
We have almost completed our data collection for this program. We had three clear nights with the Low-resolution Spectrometer at the Keck 10m telescope, three nights with the wide-field imager and three nights with the low-resolution spectrometer at the 6.5m MMT telescope, three nights with the KPNO 4m MOSAIC imager and four nights with the CTIO 4m MOSAIC-II imager. We have two additional nights scheduled with LRIS at Keck during which we hope to complete the spectroscopic survey in the clusters NGC 2168 and M34 -- these are the richest and perhaps most interesting clusters in our sample.
A UC Santa Cruz graduate student (Kate Rubin) began working on this project this year and will continue fulltime this summer (2005). This is her Masters Thesis program. She has come up to speed on the spectral reductions and completed reduction and analysis of all the Keck spectra taken to date. She is now reducing the photometric data for all the clusters. We have complete data (photometric and spectra of all WD candidates) in several clusters and are preparing a ``prototype'' paper using all our data in NGC 6633. Our goal is to use our new photometry, including 2MASS data, and two sets of stellar model tracks to rederive reddening, distance modulus and cluster age, then carry out our evaluation of the WD population. We are essentially establishing a data reduction, analysis and paper writing ``pipeline'' for the project and anticipate that we will quickly processes our results in the next 12 - 18 months.
We now have complete spectroscopic and photometric observations for most of the clusters in our sample including: IC4665, IC4725, IC4756, NGC6633, NGC6716, NGC7039, NGC7063, NGC7092, NGC7243 and Trumpler 2. For the two most important clusters in the sample, NGC2168 and M34 we have observed approximately 3/4 of the 55 and 45 candidate WDs respectively.
Activities June 2007—June 2008
We continued our program of following up photometric candidate WDs in the directions of open clusters with turnoff masses between 4 and 9 solar masses. The Low Resolution Imaging Spectrometer at the Keck 1 telescope has been used to obtain spectra of candidates in M34 (NGC 1039) and M35 (NGC 2168). We also successfully observed two LRIS masks over two nights in the older cluster M67. This has been a major program at the Keck telescope with 18 scheduled nights (and 16 nights during which data have been obtained).
We have continued to refine the Balmer-line-based analysis procedures to derive the effective temperature and log(g) of each star. These quantities can then be used to estimate the distance modulus (and cluster membership), cooling age and mass of each white dwarf. We now have final reductions for all the Keck spectra. Analysis is complete for NGC 6633, NGC 7063 M34 and M35. New results were presented at three conferences and the results for three of these four clusters were written up and published.
Three papers were completed and submitted for publication, two have been accepted at the time of this report.