101 research outputs found
The Dearth of UV-Bright Stars in M32: Implications for Stellar Evolution Theory
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope,
we have obtained deep far-ultraviolet images of the compact elliptical galaxy
M32. When combined with earlier near-ultraviolet images of the same field,
these data enable the construction of an ultraviolet color-magnitude diagram of
the hot horizontal branch (HB) population and other hot stars in late phases of
stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the
galaxy, implying that these stars either cross the HR diagram more rapidly than
expected, and/or that they spend a significant fraction of their time
enshrouded in circumstellar material. The predicted luminosity gap between the
hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected,
especially when compared to evolutionary tracks with enhanced helium
abundances, implying that the presence of hot HB stars in this metal-rich
population is not due to (Delta Y)/(Delta Z) > 4. Only a small fraction (~2%)
of the HB population is hot enough to produce significant UV emission, yet most
of the UV emission in this galaxy comes from the hot HB and AGBM stars,
implying that PAGB stars are not a significant source of UV emission even in
those elliptical galaxies with a weak UV excess.Comment: Accepted for publication in The Astrophysical Journal. Latex, 18
pages, 18 black & white figures, in emulate-ApJ format. Figures 11 & 16 have
been degraded due to size constraints; the high-quality version of the paper
is at http://www.stsci.edu/~tbrown/research/m32fuv.pd
HST Snaphot Study of Variable Stars in Globular Clusters: Inner Region of NGC 6441
[Abridged] We present the results of a Hubble Space Telescope snapshot
program to survey the inner region of the globular cluster NGC 6441 for its
variable stars. A total of 57 variable stars was found including 38 RR Lyrae
stars, 6 Population II Cepheids, and 12 long period variables. Of the RR Lyrae
stars observed in this survey, 26 are pulsating in the fundamental mode with a
mean period of 0.753d and 12 are first-overtone mode pulsators with a mean
period of 0.365d. These values match up very well with those found in
ground-based surveys. Combining all the available data for NGC 6441, we find
mean periods of 0.759d and 0.375d for the RRab and RRc stars, respectively. We
also find that the RR Lyrae in this survey are located in the same regions of a
period-amplitude diagram as those found in ground-based surveys. Although NGC
6441 is a metal-rich globular cluster, its RR Lyrae more closely resemble those
in Oosterhoff type II globular clusters. However, even compared to typical
Oosterhoff type II systems, the mean period of its RRab stars is unusually
long. We also derived I-band period-luminosity relations for the RR Lyrae
stars. Of the six Population II Cepheids, five are of W Virginis type and one
is a BL Herculis variable stars. This makes NGC 6441, along with NGC 6388, the
most metal-rich globular cluster known to contain these types of variable
stars. Another variable, V118, may also be a Population II Cepheid given its
long period and its separation in magnitude from the RR Lyrae stars. We argue
that there does not appear to be a change in the period-luminosity relation
slope between the BL Herculis and W Virginis stars, but that a change of slope
does occur when the RV Tauri stars are added to the period-luminosity relation.Comment: 28 pages, including 9 figures and 8 tables, emulateapj5/apjfonts
style. Accepted by the Astronomical Journal. Approximate publication date
September 2003. We recommend the interested reader to download the preprint
with full-resolution figures, which can be found at
http://www.astro.puc.cl/~mcatelan/Pritzl.zi
Red giant bound on the axion-electron coupling reexamined
If axions or other low-mass pseudoscalars couple to electrons (``fine
structure constant'' ) they are emitted from red giant stars by the
Compton process and by bremsstrahlung .
We construct a simple analytic expression for the energy-loss rate for all
conditions relevant for a red giant and include axion losses in evolutionary
calculations from the main sequence to the helium flash. We find that
\alpha_a\lapprox0.5\mn(-26) or m_a\lapprox 9\,\meV/\cos^2\beta lest the red
giant core at helium ignition exceed its standard mass by more than
0.025\,\MM_\odot, in conflict with observational evidence. Our bound is the
most restrictive limit on , but it does not exclude the possibility
that axion emission contributes significantly to the cooling of ZZ~Ceti stars
such as G117--B15A for which the period decrease was recently measured.Comment: 11 pages, uuencoded and compressed postscript fil
Design and Performance of SiPM-Based Readout of PbF\u3csub\u3e2\u3c/sub\u3e Crystals for High-Rate, Precision Timing Applications
We have developed a custom amplifier board coupled to a large-format 16-channel Hamamatsu silicon photomultiplier device for use as the light sensor for the electromagnetic calorimeters in the Muon g - 2 experiment at Fermilab. The calorimeter absorber is an array of lead-fluoride crystals, which produces short-duration Cherenkov light. The detector sits in the high magnetic field of the muon storage ring. The SiPMs selected, and their accompanying custom electronics, must preserve the short pulse shape, have high quantum efficiency, be non-magnetic, exhibit gain stability under varying rate conditions, and cover a fairly large fraction of the crystal exit surface area. We describe an optimized design that employs the new-generation of thru-silicon via devices. The performance is documented in a series of bench and beam tests
Improved Color-Temperature Relations and Bolometric Corrections for Cool Stars
We present new grids of colors and bolometric corrections for F-K stars
having 4000 K < Teff < 6500 K, 0.0 < log g < 4.5 and -3.0 < [Fe/H] < 0.0. A
companion paper extends these calculations into the M giant regime. Colors are
tabulated for Johnson U-V and B-V; Cousins V-R and V-I; Johnson-Glass V-K, J-K
and H-K; and CIT/CTIO V-K, J-K, H-K and CO. We have developed these
color-temperature (CT) relations by convolving synthetic spectra with
photometric filter-transmission-profiles. The synthetic spectra have been
computed with the SSG spectral synthesis code using MARCS stellar atmosphere
models as input. Both of these codes have been improved substantially,
especially at low temperatures, through the incorporation of new opacity data.
The resulting synthetic colors have been put onto the observational systems by
applying color calibrations derived from models and photometry of field stars
which have Teffs determined by the infrared-flux method. The color calibrations
have zero points and slopes which change most of the original synthetic colors
by less than 0.02 mag and 5%, respectively. The adopted Teff scale (Bell &
Gustafsson 1989) is confirmed by the extraordinary agreement between the
predicted and observed angular diameters of the field stars. We have also
derived empirical CT relations from the field-star photometry. Except for the
coolest dwarfs (Teff < 5000 K), our calibrated, solar-metallicity model colors
are found to match these and other empirical relations quite well. Our
calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to
color-magnitude diagrams of M67. We regard this as evidence that our calibrated
colors can be applied to many astrophysical problems, including modelling the
integrated light of galaxies. (abridged)Comment: To appear in the March 2000 issue of the Astronomical Journal. 72
pages including 16 embedded postscript figures (one page each) and 6 embedded
postscript tables (18 pages total
Studies of an array of PbF2 Cherenkov crystals with large-area SiPM readout
The electromagnetic calorimeter for the new muon (g-2) experiment at Fermilab
will consist of arrays of PbF2 Cherenkov crystals read out by large-area
silicon photo-multiplier (SiPM) sensors. We report here on measurements and
simulations using 2.0 -- 4.5 GeV electrons with a 28-element prototype array.
All data were obtained using fast waveform digitizers to accurately capture
signal pulse shapes versus energy, impact position, angle, and crystal
wrapping. The SiPMs were gain matched using a laser-based calibration system,
which also provided a stabilization procedure that allowed gain correction to a
level of 1e-4 per hour. After accounting for longitudinal fluctuation losses,
those crystals wrapped in a white, diffusive wrapping exhibited an energy
resolution sigma/E of (3.4 +- 0.1) % per sqrt(E/GeV), while those wrapped in a
black, absorptive wrapping had (4.6 +- 0.3) % per sqrt(E/GeV). The
white-wrapped crystals---having nearly twice the total light
collection---display a generally wider and impact-position-dependent pulse
shape owing to the dynamics of the light propagation, in comparison to the
black-wrapped crystals, which have a narrower pulse shape that is insensitive
to impact position.Comment: 14 pages, 19 figures, accepted to Nucl.Instrum.Meth. A. In v2, edited
Figures 14,15, and 17 for clarity, improved explanation of energy resolution
systematics, added reference to SiP
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Asteroseismology of old open clusters with Kepler: direct estimate of the integrated RGB mass loss in NGC6791 and NGC6819
Mass loss of red giant branch (RGB) stars is still poorly determined, despite
its crucial role in the chemical enrichment of galaxies. Thanks to the recent
detection of solar-like oscillations in G-K giants in open clusters with
Kepler, we can now directly determine stellar masses for a statistically
significant sample of stars in the old open clusters NGC6791 and NGC6819. The
aim of this work is to constrain the integrated RGB mass loss by comparing the
average mass of stars in the red clump (RC) with that of stars in the
low-luminosity portion of the RGB (i.e. stars with L <~ L(RC)). Stellar masses
were determined by combining the available seismic parameters numax and Dnu
with additional photometric constraints and with independent distance
estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of
the old metal-rich cluster NGC6791. We find that the difference between the
average mass of RGB and RC stars is small, but significant (Delta M=0.09 +-
0.03 (random) +- 0.04 (systematic) Msun). Interestingly, such a small DeltaM
does not support scenarios of an extreme mass loss for this metal-rich cluster.
If we describe the mass-loss rate with Reimers' prescription, a first
comparison with isochrones suggests that the observed DeltaM is compatible with
a mass-loss efficiency parameter in the range 0.1 <~ eta <~ 0.3. Less stringent
constraints on the RGB mass-loss rate are set by the analysis of the ~ 2
Gyr-old NGC6819, largely due to the lower mass loss expected for this cluster,
and to the lack of an independent and accurate distance determination. In the
near future, additional constraints from frequencies of individual pulsation
modes and spectroscopic effective temperatures, will allow further stringent
tests of the Dnu and numax scaling relations, which provide a novel, and
potentially very accurate, means of determining stellar radii and masses.Comment: 13 pages, 7 figures, accepted for publication in MNRA
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
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