By Annalisa Calamida, Postdoctoral Fellow at STScI
The Milky Way bulge is the closest galaxy bulge that can be observed and studied in detail. The bulge could include as much as a quarter of the stellar mass of the Milky Way (Sofue et al. 2009), and the characterization of its properties can provide crucial information for the understanding of the formation of the Galaxy and similar, more distant galaxies. We observed the Sagittarius low-reddening window, a region of relative low extinction in the bulge, E(B-V) ≤ 0.6 mag (Oosterhoff & Ponsen 1968), with the Wide-Field Channel of the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST). The time-series images were collected in the F606W and F814W filters and cover three seasons of observations from October 2011 to October 2014. The main goal of the project led by Dr. Kailash Sahu is to detect isolated stellar mass black holes and neutron stars in the Galactic bulge and disk through gravitational microlensing.
However, these data, covering 8 WFC3 and 4 ACS fields and including a sample of about 2 million stars, are a gold mine for other Galactic bulge stellar population studies. I started a project aimed at characterizing the bulge stellar populations through the study of different evolutionary phases. The work is based on the same data set used for the microlensing project and on observations taken with ACS in 2004. By combining the observations of one of the ACS fields with those taken in 2004, we measured very precise proper motions, better than ~ 0.5 mas/yr (~ 20 Km/s) at F606W ~ 28 mag, in both axes. Proper motions allowed us to separate disk and bulge stars and to obtain the deepest clean color-magnitude diagram of the bulge. As a consequence we identified for the first time a clearly defined white dwarf cooling sequence in the Galactic bulge (Calamida et al. 2014).
The characterization of the white dwarf population is an effective method to understand the formation history of the bulge and the Milky Way itself, since most stars end their life as white dwarfs. The white dwarf population of the bulge contains important information on the early star formation history of the Milky Way. Our knowledge is most extensive for the white dwarf population of the Galactic disk, in which numerous white dwarfs were discovered through imaging surveys, such as the Sloan Digital Sky Survey (Eisenstein et al. 2006, Kepler et al. 2007, Kleinman et al. 2013). White dwarfs have been identified and studied in a few close Galactic globular clusters too, thanks to deep imaging with HST (Richer et al. 2004; Hansen et al. 2007; Kalirai et al. 2007; Calamida et al. 2008; Bedin et al. 2009). Many of the Milky Way bulge stars are metal rich – several of them even reaching super solar metallicity – which means that they are similar to the Galactic disk population, and the bulge stellar space density is closer to that of the disk than that of globular clusters. However, many of the bulge stars are old, like globular cluster stars, which means that they show common properties with both the disk and the cluster populations. Understanding whether the white dwarfs in the Galactic bulge more closely resemble those found in either the disk or the clusters is therefore an important part of developing our understanding of how the Milky Way was formed, as well as the bulge formation, and, indeed, the nature of the Galactic bulge itself.
The color-magnitude-diagram in the F606W and F814W filters of selected bulge stars is shown in Fig.1, where confirmed white dwarfs are marked with larger filled dots. We used theoretical cooling tracks from the BaSTI database (Pietrinferni et al. 2004; 2006) and models from Althaus et al. (2009) to fit the bulge white dwarf cooling sequence, and a distance modulus DM0 = 14.45 mag and reddening E(B-V) = 0.5 mag (Sahu et al. 2006). These models assume different core and atmospheric compositions: CO-core white dwarfs with an hydrogen-rich envelope (DA), CO-core white dwarfs with an helium-rich envelope (DB) and He-core white dwarfs. Assuming an age of about 11 Gyr and an average solar chemical composition for bulge stars, isochrones predict a turn-off mass of ~ 0.95 M☉, and through the initial-to-final mass relationship, a white dwarf mass of ~ 0.53 – 0.55 M☉ (Weiss & Ferguson 2009, Salaris et al. 2010). The figure below shows cooling tracks for DA (dashed blue line) and DB (dashed green) CO-core white dwarfs with mass M = 0.54 M☉ and He-core white dwarfs (dashed red) with mass M = 0.23 M☉ plotted on the Galactic bulge cooling sequence. The DA and DB CO-core cooling tracks are unable to reproduce the entire color range of the observed white dwarf cooling sequence. An increase in the mean mass of the white dwarfs would move these models towards bluer colors, further increasing the discrepancy. The lower mass He-core cooling track for M = 0.23 M☉ fits the red side of the bulge white dwarf sequence (note that empirical evidence shows that the lower mass limit for white dwarfs is ~ 0.2 M☉, Kepler et al. 2007). These results support the presence of a significant fraction (~ 30%) of low-mass (M ≤ 0.45 M☉) He-core white dwarfs in the Galactic bulge. According to stellar evolution models, in a Hubble time, these low-mass white dwarfs can only be produced by stars experiencing extreme mass loss events, such as in compact binaries. Among the brighter very red white dwarfs we found indeed one ellipsoidal variable (marked with a blue dot in Fig.1), probably composed of a white dwarf accreting from a main-sequence companion, and two dwarf novae (magenta dots). The fainter counterparts of these binaries could populate the region where the reddest white dwarfs are observed in the color-magnitude diagram. These systems could be composed of a white dwarf and a low-mass (M < 0. 3M☉) main-sequence companion. This hypothesis is further supported by the finding of five cataclysmic variable candidates in the same field (green dots).
Figure 1: proper-motion cleaned bulge color-magnitude diagram in the F606W and F814W filters. White dwarfs are marked with larger filled dots. Dashed lines display cooling tracks for CO- and He-core white dwarfs. The ellipsoidal variable and the dwarf novae are marked with blue and magenta dots, respectively, while green dots mark cataclysmic variable candidates. Error bars are also labeled.
Our observational campaign ended in November 2014 and now, by adding the third season of observations and including all the twelve ACS and WFC3 fields, we will be able to increase our sample of stars by one order of magnitude. The increased statistics will allow us to better constrain the nature of the white dwarf population in the bulge, for instance, through comparing white dwarf and main-sequence star counts.
In the future, this study will greatly benefit from the advent of the James Webb Space Telescope (JWST), which will have an improved sensitivity and spatial resolution compared to HST. Moreover, JWST will observe in the near-infrared, where the extinction is a factor of ten lower compared to the optical. The new telescope will allow us to observe the bulge white dwarfs in the near-infrared, something that is now barely feasible with HST. The white dwarf cooling sequence will then be used to estimate the age of stellar populations in the bulge, to be compared to estimates obtained by adopting other diagnostics, such as the main-sequence turn-off. This study will be fundamental for constraining the presence of an age spread in the Galactic bulge, which is now a hotly debated topic.
For more details see Calamida et al. 2014, ApJ, 790, 164
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