606 research outputs found
Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238 and
DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central
emission, surrounded by a faint shell, is present in both remnants. The central
emission has an entirely thermal spectrum dominated by strong Fe L-shell lines,
with the deduced Fe abundance in excess of solar and not consistent with the
LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and
DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission
originates in gas swept up and heated by the blast wave. A standard Sedov
analysis implies about 50 solar masses in both swept-up shells, SNR ages
between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and
subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich
supernova ejecta are close to collisional ionization equilibrium. Their
presence is unexpected, because standard Type Ia SNR models predict faint
ejecta emission with short ionization ages. Both SNRs belong to a previously
unrecognized class of Type Ia SNRs characterized by bright interior emission.
Denser than expected ejecta and/or a dense circumstellar medium around the
progenitors are required to explain the presence of Fe-rich ejecta in these
SNRs. Substantial amounts of circumstellar gas are more likely to be present in
explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar
SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr
old) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres
Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals
A "jump" in the BHB distribution in the V, u-y CMD was recently detected in
the GC M13. It is morphologically best characterized as a discontinuity in u,
u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically
from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs.
We present u, y photometry of 14 GCs obtained with 3 different telescopes
(Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose
HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB,
and other GC parameters. We suggest that the u-jump is a ubiquitous feature,
intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the
ubiquitous nature of the u-jump and the problem of low measured gravities among
BHB stars. We note that the "logg-jump" occurs over the same temperature range
as the u-jump, and that it occurs in every metal-poor GC for which gravities
have been determined--irrespective of [Fe/H], mixing history on the RGB, or any
other GC parameters. Furthermore, the u-jump and the logg-jump are connected on
a star-by-star basis. The two are likely different manifestations of the same
physical phenomenon. We present a framework which may simultaneously account
for the u-jump and the logg-jump. Reviewing spectroscopic data for several
field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence
that radiative levitation of heavy elements takes place at Teff>11,500 K,
dramatically enhancing their abundances in the atmospheres of BHB stars in the
"critical" temperature region. Model atmospheres taking diffusion effects into
account are badly needed, and will likely lead to better overall agreement
between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update
Evolution of two stellar populations in globular clusters II. Effects of primordial gas expulsion
We investigate the early evolution of two distinct populations of low-mass
stars in globular clusters under the influence of primordial gas expulsion
driven by supernovae to study if this process can increase the fraction of
second generation stars at the level required by observations. We analyse
N-body models that take into account the effect of primordial gas expulsion. We
divide the stars into two populations which mimic the chemical and dynamical
properties of stars in globular clusters so that second generation stars start
with a more centrally concentrated distribution. The main effect of gas
expulsion is to eject preferentially first generation stars while second
generation stars remain bound to the cluster. In the most favourable cases
second generation stars can account for 60% of the bound stars we see today. We
also find that at the end of the gas expulsion phase, the radial distribution
of the two populations is still different, so that long-term evolution will
further increase the fraction of second generation stars. The large fraction of
chemically anomalous stars is readily explainable as a second generation of
stars formed out of the slow winds of rapidly rotating massive stars if
globular clusters suffer explosive residual gas expulsion for a star formation
efficiency of about 0.33.Comment: 11 pages, accepted for publication in A&
Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 Msun Primary and an 0.7 Msun Secondary
We report the discovery of a double-lined, spectroscopic, eclipsing binary in
the Orion star-forming region. We analyze the system spectroscopically and
photometrically to empirically determine precise, distance-independent masses,
radii, effective temperatures, and luminosities for both components. The
measured masses for the primary and secondary, accurate to ~1%, are 1.01 Msun
and 0.73 Msun, respectively; thus the primary is a definitive pre-main-sequence
solar analog, and the secondary is the lowest-mass star yet discovered among
pre-main-sequence eclipsing binary systems. We use these fundamental
measurements to test the predictions of pre-main-sequence stellar evolutionary
tracks. None of the models we examined correctly predict the masses of the two
components simultaneously, and we implicate differences between the theoretical
and empirical effective temperature scales for this failing. All of the models
predict the observed slope of the mass-radius relationship reasonably well,
though the observations tend to favor models with low convection efficiencies.
Indeed, considering our newly determined mass measurements together with other
dynamical mass measurements of pre-main-sequence stars in the literature, as
well as measurements of Li abundances in these stars, we show that the data
strongly favor evolutionary models with inefficient convection in the stellar
interior, even though such models cannot reproduce the properties of the
present-day Sun.Comment: Accepted by Ap
Time-Series Photometry of Globular Clusters: M62 (NGC 6266), the Most RR Lyrae-Rich Globular Cluster in the Galaxy?
We present new time-series CCD photometry, in the B and V bands, for the
moderately metal-rich ([Fe/H] ~ -1.3) Galactic globular cluster (GC) M62 (NGC
6266). The present dataset is the largest obtained so far for this cluster, and
consists of 168 images per filter, obtained with the Warsaw 1.3m telescope at
the Las Campanas Observatory (LCO) and the 1.3m telescope of the Cerro Tololo
Inter-American Observatory (CTIO), in two separate runs over the time span of
three months. The procedure adopted to detect the variable stars was the
optimal image subtraction method (ISIS v2.2), as implemented by Alard. The
photometry was performed using both ISIS and DAOPHOT/ALLFRAME. We have
identified 245 variable stars in the cluster fields that have been analyzed so
far, of which 179 are new discoveries. Of these variables, 133 are fundamental
mode RR Lyrae stars (RRab), 76 are first overtone (RRc) pulsators, 4 are type
II Cepheids, 25 are long-period variables (LPV), 1 is an eclipsing binary, and
6 are not yet well classified. Such a large number of RR Lyrae stars places M62
among the top two most RR Lyrae-rich (in the sense of total number of RR Lyrae
stars present) GCs known in the Galaxy, second only to M3 (NGC 5272) with a
total of 230 known RR Lyrae stars. Since this study covers most but not all of
the cluster area, it is not unlikely that M62 is in fact the most RR Lyrae-rich
GC in the Galaxy. In like vein, we were also able to detect the largest sample
of LPV's known in a Galactic GC. We analyze a variety of Oosterhoff type
indicators for the cluster, and conclude that M62 is an Oosterhoff type I
system. This is in good agreement with the moderately high metallicity of the
cluster, in spite of its predominantly blue horizontal branch morphology --
which is more typical of Oosterhoff type II systems. We thus conclude that
metallicity plays a key role in defining Oosterhoff type. [abridged]Comment: 22 pages, 14 figures (emulateapj format). AJ, in pres
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Deuterium and Oxygen Toward Feige 110: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
We present measurements of the column densities of interstellar DI and OI
made with the Far Ultraviolet Spectroscopic Explorer (FUSE), and of HI made
with the International Ultraviolet Explorer (IUE) toward the sdOB star Feige
110 [(l,b) = (74.09, -59.07); d = 179 +265 -67 pc; z = -154 +57 -227 pc]. Our
determination of the DI column density made use of curve of growth fitting and
profile fitting analyses, while our OI column density determination used only
curve of growth techniques. The HI column density was estimated by fitting the
damping wings of the interstellar Ly alpha profile. We find log N(DI) = 15.47
+/- 0.06, log N(OI) = 16.73 +/- 0.10, and log N(HI) = 20.14 +0.13 -0.20 (all
errors 2 sigma). This implies D/H = (2.14 +/- 0.82) x 10^-5, D/O = (5.50 +1.64
-1.33) x 10^-2, and O/H = (3.89 +/- 1.67) x 10^-4. Taken with the FUSE results
reported in companion papers (Moos et al. 2001) and previous measurements of
the local interstellar medium, this suggests the possibility of spatial
variability in D/H for sight lines exceeding ~100 pc. This result may constrain
models which characterize the mixing time and length scales of material in the
local interstellar medium.Comment: 24 pages, 9 figures. Accepted for publication in the Astrophysical
Journal Supplement Series. Replacement corrects a typo in original submissio
The RR Lyrae Distance Scale
We review seven methods of measuring the absolute magnitude M_V of RR Lyrae
stars in light of the Hipparcos mission and other recent developments. We focus
on identifying possible systematic errors and rank the methods by relative
immunity to such errors. For the three most robust methods, statistical
parallax, trigonometric parallax, and cluster kinematics, we find M_V (at
[Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods
cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and
theoretical models both yield a broad range of possible values (0.45-0.70 and
0.45-0.65) due to systematic uncertainties in the temperature scale and input
physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but
this may be due to a difference in the metallicity scales of the cluster giants
and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67
+/- 0.13 and is potentially very robust, but at present is too new to be fully
tested for systematics. If the three most robust methods are combined with
Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at
[Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
We discuss the impact of Population II and Globular Cluster (GCs) stars on
the derivation of the age of the Universe, and on the study of the formation
and early evolution of galaxies, our own in particular. The long-standing
problem of the actual distance scale to Population II stars and GCs is
addressed, and a variety of different methods commonly used to derive distances
to Population II stars are briefly reviewed. Emphasis is given to the
discussion of distances and ages for GCs derived using Hipparcos parallaxes of
local subdwarfs. Results obtained by different authors are slightly different,
depending on different assumptions about metallicity scale, reddenings, and
corrections for undetected binaries. These and other uncertainties present in
the method are discussed. Finally, we outline progress expected in the near
future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty
LaTeX style file, enclose
Measurement of the K_L \to \pi\mu\nu form factor parameters with the KLOE detector
Using 328 pb^{-1}of data collected at DAFNE corresponding to 1.8
million decays, we have measured the form factor
parameters. The structure of the vector-current provides information
about the dynamics of the strong interaction; its knowledge is necessary for
evaluation of the phase-space integral required for measuring the CKM matrix
element and for testing lepton universality in kaon decays. Using a
new parametrization for the vector and scalar form factors, we find
=\pt(25.7\pm 0.6),-3, and =\pt(14.0\pm 2.1),-3,. Our
result for , together with recent lattice calculations of ,
and , satisfies the Callan-Trieman relatio
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