17 research outputs found
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 the Canada-France-Hawaii Telescope, we have now obtained spectroscopic observations of candidate WDs using both the Gemini Multi-Object Spectrograph on Gemini North and the Low-Resolution Imaging Spectrometer on Keck. Of our 24 WD candidates (all fainter than V = 22.4), 21 are spectroscopically confirmed to be bona fide WDs, four or five 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 Mâ), as expected given the cluster\u27s young age (650 Myr) and, hence, high turnoff mass (~2.4 Mâ). 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 Mâ 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
The CFHT Open Star Cluster Survey II -- Deep CCD Photometry of the Old Open Star Cluster NGC 6819
We present analysis of deep CCD photometry for the very rich, old open star
cluster NGC 6819. These CFH12K data results represent the first of nineteen
open star clusters which were imaged as a part of the CFHT Open Star Cluster
Survey. We find a tight, very rich, main-sequence and turn-off consisting of
over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD).
Main-sequence fitting of the un-evolved cluster stars with the Hyades star
cluster yields a distance modulus of (m-M)v = 12.30 +/- 0.12, for a reddening
of E(B-V) = 0.10. These values are consistent with a newly calculated
theoretical stellar isochrone of age 2.5 Gyrs, which we take to be the age of
the cluster. Detailed star counts indicate a much larger cluster extent (R =
9.5' +/- 1.0'), by a factor of ~2 over some previous estimates. Incompleteness
tests confirm a slightly negatively sloped luminosity function extending to
faint (V ~ 23) magnitudes which is indicative of a dynamically evolved cluster.
Further luminosity function and mass segregation tests indicate that low mass
objects (M < 0.65Mo) predominate in the outer regions of the cluster, 3.5 < R <
9.5. The estimation of the number of white dwarfs in NGC 6819 are in good
agreement with the observed number. For those white dwarf candidates which pass
both a statistical and image classification tests, we show comparisons to white
dwarf isochrones and cooling models which suggest the need for spectroscopy to
confirm the white dwarf nature of the brighter objects.Comment: 15 Figures and 6 Tables available in higher resolution from ADS.
Accepted for Publication in AJ -- Kalirai et al. 2001b, 122, 266. Update
The CFHT Open Star Cluster Survey I -- Cluster Selection and Data Reduction
We present this paper in conjuction with the following as the first results
in the CFHT Open Star Cluster Survey. This survey is a large BVR imaging data
set of 19 open star clusters in our Galaxy. This data set was taken with the
CFH12K mosaic CCD (42' X 28') and the majority of the clusters were imaged
under excellent photometric, sub-arcsecond seeing conditions. The combination
of multiple exposures extending to deep (V ~ 25) magnitudes with short (< 10
second) frames allows for many different studies. The primary aim of this
survey is to catalogue the white dwarf stars in these clusters and establish
observational constraints on the initial-final mass relationship for these
stars and the upper mass limit to white dwarf production. Additionally, we hope
to better determine the properties of the clusters, such as age and distance,
and also test evolution and dynamical theories by analyzing luminosity and mass
functions. In order to more easily incorporate this data in further studies, we
have produced a catalogue of positions, magnitudes, colors, and stellarity
confidence for all stars in each cluster of the survey. This paper is intended
both as a source for the astronomical community to obtain information on the
clusters in the survey and as a detailed reference of reduction procedures for
further publications of individual clusters. We discuss the methods employed to
reduce the data and compute the photometric catalogue. The scientific results
for each individual cluster and global results from the study of the entire
survey will be presented in further publications (results for the rich old open
cluster NGC 6819 immediately follow this paper).Comment: 4 Figures and 3 Tables available in higher resolution at ADS.
Accepted for Publication in AJ -- Kalirai et al. 2001a, 122, 257. Update
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
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 CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)
We continue our study of rich Galactic clusters by presenting deep CCD
observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found
to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC
2323 contains approximately 2100 stars brighter than our photometric limit of V
= 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The
color-magnitude diagrams for the clusters exhibit clear main sequences
stretching over 14 magnitudes in the V, B-V plane. Comparing these long main
sequences with those of earlier clusters in the survey, as well as with the
Hyades, has allowed for accurate distances to be established for each cluster
(dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc).
Analysis of the luminosity and mass functions suggest that despite their young
ages, both clusters are somewhat dynamically relaxed exhibiting signs of
mass-segregation. This is especially interesting in the case of NGC 2323, which
has an age of only 1.3 times the dynamical relaxation time. The present
photometry is also deep enough to detect all of the white dwarfs in both
clusters. We discuss some interesting candidates which may be the remnants of
quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC
2168 is found to be in good agreement with the main-sequence turn-off age.
These objects are potentially very important for setting constraints on the
white dwarf initial-final mass relationship and upper mass limit for white
dwarf production.Comment: 34 pages, including 12 diagrams and 5 tables. Accepted for
publication in AJ. Minor typos correcte
The CFHT open star cluster survey
We have used the Canada-France-Hawaii Telescope to obtain deep and accurate
multi-colour photometry of 19 open star clusters in our Galaxy. The
quality and size of the data set are unprecedented when compared with previous
studies of open star clusters. The clusters in the survey differ in richness,
age, metal content and location in the Galaxy and therefore provide a very
diverse database for furthering the research in several areas of astrophysics.
Some of the key issues which we address include the comparisons between the
observational results and up-to-date theoretical stellar evolutionary models,
star cluster dynamics, distance and age determinations, chemical evolution
scenarios, and star formation histories. Of particular interest is the study of
the properties of white dwarf stars, such as the initial-final mass relationship
and the upper mass limit to production. The depth gained in the present survey
(limiting V magnitude ~ 25) has allowed us to establish a large catalogue
of white dwarf candidates, and our current spectroscopic investigations will
provide the much needed observational constraints to white dwarf theoretical
models.
The colour-magnitude diagrams for the rich, young clusters in the survey
possess very tightly constrained, long main-sequences and show a much richer
cluster population than previous studies. This thesis summarizes how this was
done as well as key results for the two richest clusters in the survey, NGC 6819
and NGC 2099. For NGC 6819 we find: distance = 2500 pc, age = 2.5 Gyrs,
size = 9.5', and mass = 2600 Mâ. The cluster is found to be dynamically
evolved and exhibits mass-segregation effects. A strong cooling trail of white
dwarfs is found to be in excellent agreement with a 0.70 Mâ theoretical white
dwarf cooling model. For NGC 2099 we find: distance = 1500 pc, age = 0.5
Gyrs, size = 13.9', and mass = 2500 Mâ. The cluster exhibits some signs
of dynamical evolution, although not as severe as for NGC 6819. The white
dwarf cooling age of NGC 2099 is found to be in excellent agreement with
the turn-off age.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
Astrophysics with white dwarfs
White dwarfs are the end products of the entire stellar evolutionary process in all intermediate
and low mass stars. Over 99% of all stars in our Galaxy will eventually end
their lives as white dwarfs. Observationally, studying white dwarfs has proven to be very
difficult, primarily due to the faintness of the objects. Bright white dwarfs with M[sub v] =
11 have a luminosity only l/300th of the Sun's intrinsic brightness, while the faintest
white dwarfs are 100,000x fainter than the Sun. In this thesis, we describe three related
projects aimed at better understanding white dwarfs themselves, as well as their role as
inhabitants of our Galaxy. The data that we have acquired to study these faint stars are
of unprecedented quality and depth, thereby making possible several scientific results that
have eluded investigation in decades of previous effort. First, we provide new insight into
one of the most important questions in astrophysics today, what is the nature of the dark
matter? Specifically, we are able to marginally rule out the most likely candidates based
on microlensing results, namely white dwarfs, as a strong contribution to the dark matter.
This study represents the deepest ever look into the Galactic halo and uses Hubble Space
Telescope (HST) data. Secondly, we present results from the continuing study of open star
clusters in the Canada France Hawaii Telescope (CFHT) Open Star Cluster Survey. This
work has improved the quality of the photometry of open star clusters by over an order of
magnitude compared to what had been previously possible. We present our findings for
two very young clusters, NGC 2168 (M35) and NGC 2323 (M50), including a study of their
white dwarf populations. These two clusters, and the white dwarfs that we have found
within them, will prove to be crucial in constraining one of the most fundamental relations
in stellar evolution, the initial-final mass relationship. In the third project, we use the
8-metre Gemini North and 10-metre Keck telescopes to simultaneously obtain spectra for
22 white dwarfs in the rich cluster NGC 2099. This work represents a planned follow-up
study of the white dwarfs in the richest clusters that we identified in the CFHT Open
Star Cluster Survey, and has produced several interesting results. First, all white dwarfs
in this cluster are hydrogen rich suggesting perhaps that the ratio of hydrogen to helium
white dwarfs is different in clusters than in the field, or that all massive white dwarfs are
hydrogen rich. Secondly, the NGC 2099 white dwarfs provide the first ever confirmation
of a white dwarf cooling age for a star cluster. Thirdly, with just this one cluster, we are able to almost double the number of white dwarfs that exist on the initial-final mass
plane, and provide very strong, tight constraints on a key part of the initial-final mass
relationship. The previous constraints on this relationship, which show a large scatter,
had taken over 30 years to establish. Our findings directly show that stars with masses
between 2.8-3.5 M lose 75% of their mass through stellar evolution. Additionally, for
the first time, we are beginning to see the effects of metallicity on the initial-final mass
relationship. Finally, we are now in a position to obtain further spectroscopy of white
dwarfs in the other rich clusters that we have imaged with CFHT, and, in the very near
future, plan to put over 100 data points on the initial-final mass plane.Science, Faculty ofPhysics and Astronomy, Department ofGraduat