113 research outputs found
The helium abundances in HgMn and normal stars
The parameter-free model of diffusion in the atmospheres of HgMn stars
(Michaud 1986; Michaud et al 1979) predicts that helium should sink below the
He II ionization zone in order that diffusion of other elements may take place,
and that all HgMn stars should have deficits of helium in their photospheres,
with a minimum deficit of 0.3 dex. In this study, the Smith & Dworetsky (1993)
sample of HgMn stars and normal comparison stars is examined, and the helium
abundances determined by spectrum synthesis using echelle spectra taken at Lick
Observatory and the AAT. The prediction is confirmed; all HgMn stars are
deficient in He by as much as 1.5 dex. Also, two HgMn stars, HR7361 and HR7664,
show clear evidence of helium stratification.Comment: 7 pages, 6 figures, (poster paper at IAU Symposium 224, The A-Star
Puzzle, 7-13 July 2004, Poprad, Slovakia
Radiative accelerations on Ne in the atmospheres of late B stars
Radiative accelerations on Ne are calculated for the atmospheres of main
sequence stars with 11000 < Teff < 15000 K, corresponding to the range of the
HgMn stars. The calculations take into account neon fine structure as well as
shadowing of neon lines using the entire Kurucz line list, bound-bound,
bound-free, and free-free opacity of H, He, and C as well as some NLTE effects.
NLTE effects modify the radiative acceleration by a factor of order 100 in the
outer atmosphere. The dependence of the radiative acelerations on the Ne
abundance, Teff, and gravity is studied. Radiative accelerations are well below
gravity in the entire range of Teff and it is predicted that in stable
atmospheres Ne should sink and be observed as underabundant. This agrees with
recent observations of low Ne abundances in HgMn stars.Comment: Accepted by Monthly Notices of the Royal Astronomical Society, 2002
August 21. 10 pages, 9 Postscript figures (needed new version due to error in
the listed originally-received date; corrected typo in author line)
Neon abundances in normal late-B and mercury-manganese stars
We make new Non-LTE calculations to deduce abundances of neon from
visible-region echelle spectra of selected Ne I lines in 7 normal stars and 20
HgMn stars. We find that the best strong blend-free Ne line which can be used
at the lower end of the Teff range is 6402 A, although several other
potentially useful Ne I lines are found in the red region of the spectra of
these stars. The mean neon abundance in the normal stars (log A =8.10) is in
excellent agreement with the standard abundance of neon (8.08). However, in
HgMn stars, neon is almost universally underabundant, ranging from marginal
deficits of 0.1-0.3 dex to underabundances of an order of magnitude or more. In
many cases, the lines are so weak that only upper limits can be established.
The most extreme example found is upsilon Her with an underabundance of at
least 1.5 dex. These underabundances are qualitatively expected from radiative
acceleration calculations, which show that Ne has a very small radiative
acceleration in the photosphere and is expected to undergo gravitational
settling if mixing processes are sufficiently weak, and there is no strong
stellar wind. According to the theoretical predictions of Landstreet et al.
(1998), the low Ne abundances place an important constraint on the intensity of
such stellar winds, which must be less than $10^-14 M_sun per yr if they are
non-turbulent.Comment: 10 pages, 1 figure, received 23 June 2000, accepted 4 August 2000, by
Monthly Notices of the Royal Astronomical Societ
No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81
Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds
are likely white dwarfs that steadily or cyclically burn accreted matter on
their surface, which are promising type Ia supernova progenitors. Observations
of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources
that are generally hotter and more luminous, including some ultraluminous
supersoft sources (ULSs) that are possibly intermediate mass black holes of a
few thousand solar masses. In this paper we report our X-ray spectral and
timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
eV blackbody for a white dwarf, or a
eV multicolor accretion disk for a
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ( yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, the
eclipse-like dramatic flux changes in hours are hard to explain under the white
dwarf model, but can in principle be explained by disk temperature changes
induced by accretion rate variations under the intermediate mass black hole
model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ
A Search for Distant Galactic Cepheids Toward l=60
We present results of a survey of a 6-square-degree region near l=60, b=0 to
search for distant Milky Way Cepheids. Few MW Cepheids are known at distances
>~ R_0, limiting large-scale MW disk models derived from Cepheid kinematics;
this work was designed to find a sample of distant Cepheids for use in such
models. The survey was conducted in the V and I bands over 8 epochs, to a
limiting I~=18, with a total of ~ 5 million photometric observations of ~ 1
million stars. We present a catalog of 578 high-amplitude variables discovered
in this field. Cepheid candidates were selected from this catalog on the basis
of variability and color change, and observed again the following season. We
confirm 10 of these candidates as Cepheids with periods from 4 to 8 days, most
at distances > 3 kpc. Many of the Cepheids are heavily reddened by intervening
dust, some with implied extinction A_V > 10 mag. With a future addition of
infrared photometry and radial velocities, these stars alone can provide a
constraint on R_0 to 8%, and in conjunction with other known Cepheids should
provide good estimates of the global disk potential ellipticity.Comment: 18 pages, 4 tables, 13 figures (LaTeX / AASTeX
Brain hubs defined in the group do not overlap with regions of high inter-individual variability
Connector \u27hubs\u27 are brain regions with links to multiple networks. These regions are hypothesized to play a critical role in brain function. While hubs are often identified based on group-average functional magnetic resonance imaging (fMRI) data, there is considerable inter-subject variation in the functional connectivity profiles of the brain, especially in association regions where hubs tend to be located. Here we investigated how group hubs are related to locations of inter-individual variability. To answer this question, we examined inter-individual variation at group-level hubs in both the Midnight Scan Club and Human Connectome Project datasets. The top group hubs defined based on the participation coefficient did not overlap strongly with the most prominent regions of inter-individual variation (termed \u27variants\u27 in prior work). These hubs have relatively strong similarity across participants and consistent cross-network profiles, similar to what was seen for many other areas of cortex. Consistency across participants was further improved when these hubs were allowed to shift slightly in local position. Thus, our results demonstrate that the top group hubs defined with the participation coefficient are generally consistent across people, suggesting they may represent conserved cross-network bridges. More caution is warranted with alternative hub measures, such as community density (which are based on spatial proximity to network borders) and intermediate hub regions which show higher correspondence to locations of individual variability
Xenon in Mercury-Manganese Stars
Previous studies of elemental abundances in Mercury-Manganese (HgMn) stars
have occasionally reported the presence of lines of the ionized rare noble gas
Xe II, especially in a few of the hottest stars with Teff ~ 13000--15000 K. A
new study of this element has been undertaken using observations from Lick
Observatory's Hamilton Echelle Spectrograph. In this work, the spectrum
synthesis program UCLSYN has been used to undertake abundance analysis assuming
LTE. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly
over-abundant in 21 of 22 HgMn stars studied, by factors of 3.1--4.8 dex. There
does not appear to be a significant correlation of Xe abundance with Teff. A
comparison sample of normal late B stars shows no sign of Xe II lines that
could be detected, consistent with the expected weakness of lines at normal
abundance. The main reason for the previous lack of widespread detection in
HgMn stars is probably due to the strongest lines being at longer wavelengths
than the photographic blue. The lines used in this work were 4603.03A, 4844.33A
and 5292.22A.Comment: 8 pages, 4 figures. Accepted by Monthly Notices of the Royal
Astronomical Society, 8 January 200
Dwarf Cepheids in the Carina Dwarf Spheroidal Galaxy
We have discovered 20 dwarf Cepheids (DC) in the Carina dSph galaxy from the
analysis of individual CCD images obtained for a deep photometric study of the
system. These short-period pulsating variable stars are by far the most distant
(~100 kpc) and faintest (V ~ 23.0) DCs known. The Carina DCs obey a
well-defined period-luminosity relation, allowing us to readily distinguish
between overtone and fundamental pulsators in nearly every case. Unlike RR Lyr
stars, the pulsation mode turns out to be uncorrelated with light-curve shape,
nor do the overtone pulsators tend towards shorter periods compared to the
fundamental pulsators. Using the period-luminosity (PL) relations from Nemec et
al. (1994 AJ, 108, 222) and McNamara (1995, AJ, 109, 1751), we derive (m-M)_0 =
20.06 +/- 0.12, for E(B-V) = 0.025 and [Fe/H] = -2.0, in good agreement with
recent, independent estimates of the distance/reddening of Carina. The error
reflects the uncertainties in the DC distance scale, and in the metallicity and
reddening of Carina. The frequency of DCs among upper main sequence stars in
Carina is approximately 3%. The ratio of dwarf Cepheids to RR Lyr stars in
Carina is 0.13 +/- 0.10, though this result is highly sensitive to the
star-formation history of Carina and the evolution of the Horizontal Branch. We
discuss how DCs may be useful to search effectively for substructure in the
Galactic halo out to Galactocentric distances of ~100 kpc.Comment: 20 pages of text, 7 figure
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