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Register a new userOn the radial distribution of white dwarfs in the Galactic globular cluster Omega Cen
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We present deep and accurate photometry (F435W, F625W, F658N) of the Galactic Globular Cluster Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We identified ~ 6,500 white dwarf (WD)candidates and compared their radial distribution with that of Main Sequence (MS) stars. We found a mild evidence that young WDs (0.1 < t < 0.6 Gyr) are less centrally concentrated when compared to MS stars in the magnitude range 25 < F435W < 26.5.
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We present deep multiband (F435W, F625W, and F658N) photometric data of the Globular Cluster Omega Cen collected with the Advanced Camera for Surveys on board of the Hubble Space Telescope. We identified in the (F435W-F625W, F435W) plane more than two thousand White Dwarf (WD) candidates using three out of nine available pointings. Such a large sample appears in agreement with predictions based on the ratio between WD and Horizontal Branch (HB) evolutionary lifetimes. We also detected ~ 1600 WDs in the (F658N-F625W, F625W) plane, supporting the evidence that a large fraction of current cluster WDs are $H_alpha$ bright.
We present deep and precise photometry (F435, F625W, F658N) of Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We have identified ~ 6,500 white dwarf (WD) candidates, and the ratio of WD and Main Sequence (MS) star counts is found to be at least a factor of two larger than the ratio of CO-core WD cooling and MS lifetimes. This discrepancy is not explained by the possible occurrence of a He-enhanced stellar population, since the MS lifetime changes by only 15% when changing from a canonical (Y=0.25) to a He-enhanced composition (Y=0.42). The presence of some He-core WDs seems able to explain the observed star counts. The fraction of He WDs required ranges from 10% to 80% depending on their mean mass and it is at least five times larger than for field WDs. The comparison in the Color Magnitude Diagram between theory and observations also supports the presence of He WDs. Empirical evidence indicates that He WDs have been detected in stellar systems hosting a large sample of extreme horizontal branch stars, thus suggesting that a fraction of red giants might avoid the He-core flash.
We present dynamical status of the Galactic globular cluster NGC 6656 using spatial distribution of Blue Straggler Stars (BSSs). A combination of multi-wavelength high-resolution space and ground-based data are used to cover a large cluster region. We determine the centre of gravity ($C_{grav}$) and construct the projected density profile of the cluster using the probable cluster members selected from HST and Gaia DR2 proper motion data sets. The projected density profile in the investigated region is nicely reproduced by a single mass King model, with core ($r_{c}$) and tidal ($r_{t}$) radius as $75^{primeprime}.2$ $pm$ $3^{primeprime}.1$ and $35^{prime}.6$ $pm$ $1^{prime}.1$ respectively. In total, 90 BSSs are identified on the basis of proper motion data in the region of radius $625^{primeprime}$. An average mass of the BSSs is determined as $1.06$ $pm$ $0.09$ $M_{odot}$ and with an age range of 0.5 to 7 Gyrs. The BSS radial distribution shows a bimodal trend, with a peak in the centre, a minimum at $r sim r_c$ and a rising tendency in the outer region. The BSS radial distribution shows a flat behaviour in the outermost region of the cluster. We also estimate $A^{+}_{rh}$ parameter as an alternative indicator of the dynamical status of the cluster and is found to be $0.038$ $pm$ $0.016$. Based on the radial distribution and $A^{+}_{rh}$ parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster.
We present results of a study of the central regions of NGC 6397 using Hubble Space Telescopes Advanced Camera for Surveys, focusing on a group of 24 faint blue stars that form a sequence parallel to, but brighter than, the more populated sequence of carbon-oxygen white dwarfs (CO WDs). Using F625W, F435W, and F658N filters with the Wide Field Channel we show that these stars, 18 of which are newly discovered, have magnitudes and colors consistent with those of helium-core white dwarfs (He WDs) with masses ~ 0.2-0.3 Msun. Their H-alpha--R625 colors indicate that they have strong H-alpha absorption lines, which distinguishes them from cataclysmic variables in the cluster. The radial distribution of the He WDs is significantly more concentrated to the cluster center than that of either the CO WDs or the turnoff stars and most closely resembles that of the clusters blue stragglers. Binary companions are required to explain the implied dynamical masses. We show that the companions cannot be main-sequence stars and are most likely heavy CO WDs. The number and photometric masses of the observed He WDs can be understood if ~1-5% of the main-sequence stars within the half-mass radius of the cluster have white dwarf companions with orbital periods in the range ~1-20 days at the time they reach the turnoff. In contrast to the CO WD sequence, the He WD sequence comes to an end at R625 ~ 24.5, well above the magnitude limit of the observations. We explore the significance of this finding in the context of thick vs. thin hydrogen envelope models and compare our results to existing theoretical predictions. In addition, we find strong evidence that the vast majority of the CO WDs in NGC 6397 down to Teff ~ 10,000 K are of the DA class. Finally, we use the CO WD sequence to measure a distance to the cluster of 2.34 +- 0.13 kpc.
With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass sim 0.5 M_{odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond sim 3 times 10^8 years will require deeper and more complete data.
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