3,203 research outputs found
Fuel-Supply-Limited Stellar Relaxation Oscillations: Application to Multiple Rings around AGB Stars and Planetary Nebulae
We describe a new mechanism for pulsations in evolved stars: relaxation
oscillations driven by a coupling between the luminosity-dependent mass-loss
rate and the H fuel abundance in a nuclear-burning shell. When mass loss is
included, the outward flow of matter can modulate the flow of fuel into the
shell when the stellar luminosity is close to the Eddington luminosity . When the luminosity drops below , the mass outflow declines
and the shell is re-supplied with fuel. This process can be repetitive. We
demonstrate the existence of such oscillations and discuss the dependence of
the results on the stellar parameters. In particular, we show that the
oscillation period scales specifically with the mass of the H-burning
relaxation shell (HBRS), defined as the part of the H-burning shell above the
minimum radius at which the luminosity from below first exceeds the Eddington
threshold at the onset of the mass loss phase. For a stellar mass M_*\sim
0.7\Msun, luminosity L_*\sim 10^4\Lsun, and mass loss rate |\dot M|\sim
10^{-5}\Msun yr, the oscillations have a recurrence time
years , where is the timescale for
modulation of the fuel supply in the HBRS by the varying mass-loss rate. This
period agrees with the 1400-year period inferred for the spacings
between the shells surrounding some planetary nebulae, and the the predictied
shell thickness, of order 0.4 times the spacing, also agrees reasonably well.Comment: 15 pages TeX, 1 ps figure submitted to Ap
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
Flash Mixing on the White Dwarf Cooling Curve: FUSE Observations of three He-rich sdB Stars
We present FUSE spectra of 3 He-rich sdB stars. Two of these stars,
PG1544+488 and JL87, reveal extremely strong C III lines at 977 and 1176A,
while the carbon lines are quite weak in the third star, LB1766. We have
analyzed the FUSE data using TLUSTY NLTE line-blanketed model atmospheres, and
find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87
shows a similar surface enrichment of carbon and nitrogen, but some significant
fraction of hydrogen still remains in its atmosphere. LB1766 has a surface
composition devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing. We interpret these
observations with new evolutionary calculations which suggest that He-rich sdB
stars with C-rich compositions are the progeny of stars which underwent a
delayed He-core flash on the white-dwarf cooling curve. During such a flash the
interior convection zone will penetrate into the H envelope, thereby mixing the
envelope with the He- and C-rich core. Such `flash-mixed' stars will arrive on
the extreme horizontal branch (EHB) with He- and C-rich surface compositions
and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two
types of flash mixing are possible: `deep' and `shallow', depending on whether
the H envelope is mixed deeply into the site of the He flash or only with the
outer layers of the core. Based on both their stellar parameters and surface
compositions, we suggest that PG1544+488 and JL87 are examples of `deep' and
`shallow' flash mixing, respectively. Flash mixing may therefore represent a
new evolutionary channel for producing the hottest EHB stars. However, flash
mixing cannot explain the abundance pattern in LB1766, which remains a
challenge to current evolutionary models.Comment: 32 pages, 6 figures; minor additions; to appear in Astrophys. J.,
February 200
The Blue Hook Populations of Massive Globular Clusters
We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic
globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These
observations were obtained to investigate the "blue hook" phenomenon previously
observed in UV images of the globular clusters omega Cen and NGC 2808. Blue
hook stars are a class of hot (approximately 35,000 K) subluminous horizontal
branch stars that occupy a region of the HR diagram that is unexplained by
canonical stellar evolution theory. By coupling new stellar evolution models to
appropriate non-LTE synthetic spectra, we investigate various theoretical
explanations for these stars. Specifically, we compare our photometry to
canonical models at standard cluster abundances, canonical models with enhanced
helium (consistent with cluster self-enrichment at early times), and
flash-mixed models formed via a late helium-core flash on the white dwarf
cooling curve. We find that flash-mixed models are required to explain the
faint luminosity of the blue hook stars, although neither the canonical models
nor the flash-mixed models can explain the range of color observed in such
stars, especially those in the most metal-rich clusters. Aside from the
variation in the color range, no clear trends emerge in the morphology of the
blue hook population with respect to metallicity.Comment: Accepted for publication in The Astrophysical Journal. Latex, 14
pages, 1 B&W and 6 color figure
Blood glucose-lowering nuclear receptor agonists only partially normalize hepatic gene expression in db/db mice
ABSTRACT Agonists of the nuclear receptors peroxisome proliferator-activated receptor (PPAR) â„, PPARâŁ, and liver X receptors (LXRs) reduce blood glucose in type 2 diabetic patients and comparable mouse models. Since the capacity of these drugs to normalize hepatic gene expression is not known, we compared groups of obese diabetic db/db mice treated with agonists for PPARâ„ [rosiglitazone (Rosi); 10 mg/kg/day], PPAR⣠[Wy 14643 (Wy; 4-chloro-6 -(2,3-xylidino)-2-pyrimidinyl)thioacetic acid); 30 mg/kg/day], and LXR [T0901317 (T09; N-(2,2,2-trifluoroethyl)-N-[4-[2,2,2-trifluoro-1-hydroxy-1(trifluoromethyl)-ethyl]phenyl]-benzenesulfonamide) ; 40 mg/kg/day] and from untreated nondiabetic litter mates (db/Ï©) by oligonucleotide microarrays and quantitative reverse transcriptase-polymerase chain reaction. The 10-day treatment period of db/db mice with Rosi, Wy, and T09 altered expression of 300, 620, and 735 genes including agonist-specific target genes, respectively. However, from the 337 genes differentially regulated in untreated db/Ï© versus db/db animals, only 34 (10%), 51 (15%), and 82 (24%) were regulated in the direction of the db/Ï© group by Rosi, Wy, and T09, respectively. Gene expression normalization by drug treatment involved glucose homeostasis, lipid homeostasis, and local glucocorticoid activation. In addition, our data pointed to hitherto unknown interference of these nuclear receptors with growth hormone receptor gene expression and endoplasmic reticulum stress. However, many diabetes-associated gene alterations remained unaffected or were even aggravated by nuclear receptor agonist treatment. These results suggest that diabetes-induced gene expression is minimally reversed by potent blood glucose-lowering nuclear receptor agonists
Two Rare Magnetic Cataclysmic Variables with Extreme Cyclotron Features Identified in the Sloan Digital Sky Survey
Two newly identified magnetic cataclysmic variables discovered in the Sloan
Digital Sky Survey (SDSS), SDSSJ155331.12+551614.5 and SDSSJ132411.57+032050.5,
have spectra showing highly prominent, narrow, strongly polarized cyclotron
humps with amplitudes that vary on orbital periods of 4.39 and 2.6 hrs,
respectively. In the former, the spacing of the humps indicates the 3rd and 4th
harmonics in a magnetic field of ~60 MG. The narrowness of the cyclotron
features and the lack of strong emission lines imply very low temperature
plasmas and very low accretion rates, so that the accreting area is heated by
particle collisions rather than accretion shocks. The detection of rare systems
like these exemplifies the ability of the SDSS to find the lowest accretion
rate close binaries.Comment: Accepted for publication in the Astrophysical Journal, vol. 583,
February 1, 2003; slight revisions and additions in response to referee's
comments; 17 pages, 6 figures, AASTeX v4.
GALEX and Optical Light Curves of WX LMi, SDSSJ103100.5+202832.2 and SDSSJ121209.31+013627.7
{\it GALEX} near ultraviolet (NUV) and far-ultraviolet (FUV) light curves of
three extremely low accretion rate polars show distinct modulations in their UV
light curves. While these three systems have a range of magnetic fields from 13
to 70 MG, and of late type secondaries (including a likely brown dwarf in
SDSSJ121209.31+013627.7), the accretion rates are similar, and the UV
observations imply some mechanism is operating to create enhanced emission
zones on the white dwarf. The UV variations match in phase to the two magnetic
poles viewed in the optical in WX LMi and to the single poles evident in the
optical in SDSSJ1212109.31+013627.7 and SDSSJ103100.55+202832.2. Simple spot
models of the UV light curves show that if hot spots are responsible for the UV
variations, the temperatures are on the order of 10,000-14,000K. For the single
pole systems, the size of the FUV spot must be smaller than the NUV and in all
cases, the geometry is likely more complicated than a simple circular spot.Comment: 29 pages, 4 tables, 10 figures, Astrophysical Journal, accepte
Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State
The polars VV Pup and V393 Pav were observed with XMM-Newton during states of
low accretion rate with peak X-ray luminosities of ~1 x 10^30 and ~1 x 10^31
erg/s, respectively. In both polars, accretion onto the white dwarf was
extremely irregular, and the accretion rate varied by more than 1 order of
magnitude on timescales of ~1 hr. Our observations suggest that this type of
irregular accretion is a common phenomenon in polars during the low state. The
likely cause of the accretion rate fluctuations are coronal mass ejections or
solar flares on the companion star that intermittently increase the mass
transfer into the accretion stream. Our findings demonstrate that the companion
stars in cataclysmic variables possess highly active atmospheres.Comment: Accepted for publication in ApJ, 16 pages, 3 figure
The epochs of early-type galaxy formation as a function of environment
The aim of this paper is to set constraints of the epochs of early-type
galaxy formation through the 'archaeology' of the stellar populations in local
galaxies. Using our models of absorption line indices that account for variable
abundance ratios, we derive the stellar population parameters of 124 early-type
galaxies in high and low density environments. We find that all three
parameters age, metallicity, and alpha/Fe ratio are correlated with velocity
dispersion. We further find evidence for an influence of the environment on the
stellar population properties. Massive early-type galaxies in low-density
environments appear on average ~2 Gyrs younger and slightly more metal-rich
than their counterparts in high density environments. No offsets in the
alpha/Fe ratios, instead, are detected. We translate the derived ages and
alpha/Fe ratios into star formation histories. We show that most star formation
activity in early-type galaxies is expected to have happened between redshifts
3 and 5 in high density and between redshifts 1 and 2 in low density
environments. We conclude that at least 50 per cent of the total stellar mass
density must have already formed at z 1, in good agreement with observational
estimates of the total stellar mass density as a function of redshift. Our
results suggest that significant mass growth in the early-type galaxy
population below z 1 must be restricted to less massive objects, and a
significant increase of the stellar mass density between redshifts 1 and 2
should be present caused mainly by the field galaxy population. The results of
this paper further imply vigorous star formation episodes in massive objects at
z 2-5 and the presence of evolved ellipticals around z 1, both observationally
identified as SCUBA galaxies and EROs.Comment: 20 pages, 10 figures, plus appendix, accepted by Ap
A comprehensive approach to analyzing the XMM-Newton data of Seyfert 1 galaxies
Aims. We seek a comprehensive analysis of all the information provided by the
XMM-Newton satellite of the four Seyfert 1 galaxies ESO 359-G19, HE 1143-1810,
CTS A08.12, and Mrk 110, including the UV range, to characterize the different
components that are emitting and absorbing radiation in the vicinity of the
active nucleus.
Methods. The continuum emission was studied through the EPIC spectra by
taking advantage of the spectral range of these cameras. The high-resolution
RGS spectra were analyzed to characterize the absorbing and emission line
features that arise in the spectra of the sources. All these data, complemented
by information in the UV, are analyzed jointly in order to achieve a consistent
characterization of the observed features in each object.
Results. The continuum emission of the sources can be characterized either by
a combination of a power law and a black body for the weakest objects or by two
power law components for the brightest ones. The continuum is not absorbed by
neutral or ionized material in the line of sight to any of these sources. In
all of them we have identified a narrow Fe-Kalpha line at 6.4 keV. In ESO
559-G19 we also find an FeXXVI line at about 7 keV. In the soft X-rays band, we
identify only one OVII line in the spectra of HE 1143-1810 and CTS A08.12, and
two OVII-He alpha triplets and a narrow OVIII-Ly alpha emission line in Mrk
110.
Conclusions. Not detecting warm material in the line of sight to the low
state objects is due to intrinsically weaker or absent absorption in the line
of sight and not to a low signal-to-noise ratio in the data. Besides this, the
absence of clear emission lines cannot be fully attributed to dilution of those
lines by a strong continuum.Comment: 14 pages, 7 figures, 9 tables, accepted by A&
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