677 research outputs found
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
We present the first detailed study of the properties (temperatures,
gravities, and masses) of the NGC 6791 white dwarf population. This unique
stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~
0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that
exhibits both a red giant clump and a much hotter extreme horizontal branch.
Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS
spectra, suggests that most of these stars are undermassive, = 0.43 +/-
0.06 Msun, and therefore could not have formed from canonical stellar evolution
involving the helium flash at the tip of the red giant branch. We show that at
least 40% of NGC 6791's evolved stars must have lost enough mass on the red
giant branch to avoid the flash, and therefore did not convert helium into
carbon-oxygen in their core. Such increased mass loss in the evolution of the
progenitors of these stars is consistent with the presence of the extreme
horizontal branch in the CMD. This unique stellar evolutionary channel also
naturally explains the recent finding of a very young age (2.4 Gyr) for NGC
6791 from white dwarf cooling theory; helium core white dwarfs in this cluster
will cool ~3 times slower than carbon-oxygen core stars and therefore the
corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well
measured main-sequence turnoff age. These results provide direct empirical
evidence that mass loss is much more efficient in high metallicity environments
and therefore may be critical in interpreting the ultraviolet upturn in
elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys.
J. Very minor changes from first versio
The White Dwarf Cooling Sequence of NGC6397
We present the results of a deep Hubble Space Telescope (HST) exposure of the
nearby globular cluster NGC6397, focussing attention on the cluster's white
dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in
depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using
both artificial star tests and the detectability of background galaxies at
fainter magnitudes, that the cutoff is real and represents the truncation of
the white dwarf luminosity function in this cluster. We perform a detailed
comparison between cooling models and the observed distribution of white dwarfs
in colour and magnitude, taking into account uncertainties in distance,
extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model
uncertainties. After marginalising over these variables, we obtain values for
the cluster distance modulus and age of \mu_0 = 12.02 \pm 0.06 and T_c = 11.47
\pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf
initial-final mass relations are in good agreement with other independent
determinations, and the cluster age is consistent with, but more precise than,
prior determinations made using the main sequence turnoff method. In
particular, within the context of the currently accepted \Lambda CDM
cosmological model, this age places the formation of NGC6397 at a redshift z=3,
at a time when the cosmological star formation rate was approaching its peak.Comment: 56 pages, 30 figure
The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro
We establish an extragalactic, zero-motion frame of reference within the
deepest optical image of a globular star cluster, an HST 123-orbit exposure of
M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16)
intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance
of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster
based on their proper motions over the 6-year baseline between these and
previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are
also found on the same sight line using image-morphology techniques. This fixed
reference frame allows us to independently determine the fundamental Galactic
constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local
Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly,
the galaxies allow a direct measurement of M4's absolute proper motion,
mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies.
The clear separation of galaxies from stars in these deep data also allow us to
search for inner-halo white dwarfs. We model the conventional Galactic
contributions of white dwarfs along our line of sight and predict 7.9 (thin
disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at
which we can clearly delineate stars from galaxies (V = 29). An additional 2.5
objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo
objects, 70% of which are of the DA type. After considering the kinematics and
morphology of the objects in our data set, we find the number of white dwarfs
to be consistent with the predictions for each of the conventional populations.
However, we do not find any evidence for dark halo white dwarfs.Comment: 31 pages, including 6 diagrams and 2 tables. Accepted for publication
in Ap
The Space Motion of the Globular Cluster NGC 6397
As a by-product of high-precision, ultra-deep stellar photometry in the
Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able
to measure a large population of background galaxies whose images are nearly
point-like. These provide an extragalactic reference frame of unprecedented
accuracy, relative to which we measure the most accurate absolute proper motion
ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr
and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical
errors quoted for the proper motion of NGC 6397 do not include possible
unavoidable sources of systematic errors, such as cluster rotation. These are
very unlikely to exceed a few percent. We use this new proper motion to
calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and
find that the cluster has made frequent passages through the Galactic disk.Comment: 5 pages including 3 figures, accepted for publication in the
Astrophysical Journal Letters. Very minor changes in V2. typos fixe
Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function
We present the CMD from deep HST imaging in the globular cluster NGC 6397.
The ACS was used for 126 orbits to image a single field in two colors (F814W,
F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of
archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM)
clean the data. Applying the PM corrections produces a remarkably clean CMD
which reveals a number of features never seen before in a globular cluster CMD.
In our field, the main sequence stars appeared to terminate close to the
location in the CMD of the hydrogen-burning limit predicted by two independent
sets of stellar evolution models. The faintest observed main sequence stars are
about a magnitude fainter than the least luminous metal-poor field halo stars
known, suggesting that the lowest luminosity halo stars still await discovery.
At the bright end the data extend beyond the main sequence turnoff to well up
the giant branch. A populous white dwarf cooling sequence is also seen in the
cluster CMD. The most dramatic features of the cooling sequence are its turn to
the blue at faint magnitudes as well as an apparent truncation near F814W = 28.
The cluster luminosity and mass functions were derived, stretching from the
turn off down to the hydrogen-burning limit. It was well modeled with either a
very flat power-law or a lognormal function. In order to interpret these fits
more fully we compared them with similar functions in the cluster core and with
a full N-body model of NGC 6397 finding satisfactory agreement between the
model predictions and the data. This exercise demonstrates the important role
and the effect that dynamics has played in altering the cluster IMF.Comment: 43 pages including 4 tables and 12 diagrams. Figures 2 and 3 have
been bitmapped. Accepted for publication in the Astronomical Journa
The Initial-Final Mass Relationship: Spectroscopy of White Dwarfs in NGC 2099 (M37)
We present new observations of very faint white dwarfs (WDs) in the rich open
star cluster NGC 2099 (M37). Following deep, wide field imaging of the cluster
using CFHT, we have now obtained spectroscopic observations of candidate WDs
using both GMOS on Gemini and LRIS on Keck. Of our 24 WD candidates (all
fainter than V = 22.4), 21 are spectroscopically confirmed to be bona fide WDs,
4-5 of which are most likely field objects. Fitting 18 of the 21 WD spectra
with model atmospheres, we find that most WDs in this cluster are quite massive
(0.7-0.9 Msun), as expected given the cluster's young age (650 Myr) and, hence,
high turnoff mass (~2.4 Msun). We determine a new initial-final mass
relationship and almost double the number of existing data points from previous
studies. The results indicate that stars with initial masses between 2.8 and
3.4 Msun lose 70-75% of their mass through stellar evolution. For the first
time, we find some evidence of a metallicity dependence on the initial-final
mass relationship.Comment: 5 pages including 3 figures and 1 table, accepted in ApJ Letters.
Minor additions to content and typos correcte
Deep ACS Imaging in the Globular Cluster NGC6397: Dynamical Models
We present N-body models to complement deep imaging of the metal-poor
core-collapsed cluster NGC6397 obtained with the Hubble Space Telescope. All
simulations include stellar and binary evolution in-step with the stellar
dynamics and account for the tidal field of the Galaxy. We focus on the results
of a simulation that began with 100000 objects (stars and binaries), 5%
primordial binaries and Population II metallicity. After 16 Gyr of evolution
the model cluster has about 20% of the stars remaining and has reached
core-collapse. We compare the color-magnitude diagrams of the model at this age
for the central region and an outer region corresponding to the observed field
of NGC6397 (about 2-3 half-light radii from the cluster centre). This
demonstrates that the white dwarf population in the outer region has suffered
little modification from dynamical processes - contamination of the luminosity
function by binaries and white dwarfs with non-standard evolution histories is
minimal and should not significantly affect measurement of the cluster age. We
also show that the binary fraction of main-sequence stars observed in the
NGC6397 field can be taken as representative of the primordial binary fraction
of the cluster. For the mass function of the main-sequence stars we find that
although this has been altered significantly by dynamics over the cluster
lifetime, especially in the central and outer regions, that the position of the
observed field is close to optimal for recovering the initial mass function of
the cluster stars (below the current turn-off mass). More generally we look at
how the mass function changes with radius in a dynamically evolved stellar
cluster and suggest where the best radial position to observe the initial mass
function is for clusters of any age.Comment: 34 pages, 11 figures, submitted to AJ, companion paper to 0708.403
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