114 research outputs found
Observed variability in the Fraunhofer line spectrum of solar flux, 1975 - 1980
Over the five years double-pass spectrometer observations of the Sun-as-a-star revealed significant changes in line intensities. The photospheric component weakened linearly with time 0 to 2.3%. From a lack of correlation between these line weakenings and solar activity indicators like sunspots and plage, a global variation of surface properties is inferred. Model-atmosphere analysis suggests a slight reduction in the lower-photospheric temperature gradient corresponding to a 15% increase in the mixing length within the granulation layer. Chromospheric lines such as Ca II H and K, Ca II 8543 and the CN band head weaken synchronously with solar activity. Thus, the behavior of photospheric and chromospheric lines is markedly different, with the possibility of secular change for the former
A New Analysis of the O VI Emitting Nebula around KPD 0005+5106
We present observations of O VI 1032 emission around the helium white dwarf
KPD 0005+5106 obtained with the Far Ultraviolet Spectroscopic Explorer.
Previously published data, reprocessed with an updated version of the
calibration pipeline, are included along with new observations. The recent
upward revision of the white dwarf's effective temperature to 200,000 K has
motivated us to re-analyze all the data. We compare observations with
photoionization models and find that the density of the O VI nebula is about 10
cm^-3, and that the stellar flux must be attenuated by about 90% by the time it
impinges on the inner face of the nebula. We infer that this attenuation is due
to circumstellar material ejected by KPD 0005+5106 earlier in its evolution.Comment: 21 pages, including 3 figures and 2 tables. Accepted for publication
in the Astrophysical Journal. Minor change: a few uncited references remove
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
A study of lambda Bootis type stars in the wavelength region beyond 7000A
The group of lambda Bootis type stars comprises late B- to early F-type,
Population I objects which are basically metal weak, in particular the Fe group
elements, but with the clear exception of C, N, O and S. One of the theories to
explain the abundance pattern of these stars involves circumstellar or
interstellar matter around the objects. Hence, we have compiled all available
data from the literature of well established members of the lambda Bootis group
redward of 7000A in order to find evidence for matter around these objects.
Furthermore, we present unpublished ISO as well as submillimeter continuum and
CO (2-1) line measurements to complete the data set. In total, measurements for
34 (26 with data redward of 20 mu m) well established lambda Bootis stars are
available. There is evidence for an infrared excesses in six stars (HD 31295,
HD 74873, HD 110411, HD 125162, HD 198160/1 and HD 210111) and two are doubtful
cases (HD 11413 and HD 192640) resulting in a percentage of 23% (excluding the
two doubtful cases). Dust models for these objects show fractional dust
luminosities comparable to the Vega-type stars and slightly higher dust
temperatures. ISO-SWS spectroscopy for HD 125162 and HD 192640 resulted in the
detection of pure stellar HI lines ruling out an active accretion disk (as
found for several Herbig Ae/Be stars) around these objects. The submillimeter
measurements gave only upper limits for the line and continuum fluxes.Comment: 16 pages, 5 figures, accepted by A&
Reversal-free CaIIH profiles: a challenge for solar chromosphere modeling in quiet inter-network
We study chromospheric emission to understand the temperature stratification
in the solar chromosphere. We observed the intensity profile of the CaIIH line
in a quiet Sun region close to the disk center at the German Vacuum Tower
Telescope. We analyze over 10^5 line profiles from inter-network regions. For
comparison with the observed profiles, we synthesize spectra for a variety of
model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative
transfer code. A fraction of about 25% of the observed CaIIH line profiles do
not show a measurable emission peak in H_{2v} and H_{2r} wavelength bands
(reversal-free). All of the chosen model atmospheres with a temperature rise
fail to reproduce such profiles. On the other hand, the synthetic calcium
profile of a model atmosphere that has a monotonic decline of the temperature
with height shows a reversal-free profile that has much lower intensities than
any observed line profile. The observed reversal-free profiles indicate the
existence of cool patches in the interior of chromospheric network cells, at
least for short time intervals. Our finding is not only in conflict with a
full-time hot chromosphere, but also with a very cool chromosphere as found in
some dynamic simulations.Comment: 8 pages, accepted in A&
The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
The edge-on disk surrounding the nearby young star Beta Pictoris is the
archetype of the "debris disks", which are composed of dust and gas produced by
collisions and evaporation of planetesimals, analogues of Solar System comets
and asteroids. These disks provide a window on the formation and early
evolution of terrestrial planets. Previous observations of Beta Pic concluded
that the disk gas has roughly solar abundances of elements [1], but this poses
a problem because such gas should be rapidly blown away from the star, contrary
to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we
report the detection of singly and doubly ionized carbon (CII, CIII) and
neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these
abundant volatile species permits a much more complete gas inventory. Carbon is
extremely overabundant relative to every other measured element. This appears
to solve the problem of the stable gas disk, since the carbon overabundance
should keep the gas disk in Keplerian rotation [3]. New questions arise,
however, since the overabundance may indicate the gas is produced from material
more carbon-rich than the expected Solar System analogues.Comment: Accepted for publication in Nature. PDF document, 12 pages.
Supplementary information may be found at
http://www.dtm.ciw.edu/akir/Documents/roberge_supp.pdf *** Version 2 :
Removed extraneous publication information, per instructions from the Nature
editor. No other changes mad
Widespread atomic gas emission reveals the rotation of the beta Pictoris disk
We present high resolution Na I D spectroscopy of the beta Pic disk, and the
resonantly scattered sodium emission can be traced from less than 30 AU to at
least 140 AU from the central star. This atomic gas is co-existent with the
dust particles, suggestive of a common origin or source. The disk rotates
towards us in the south-west and away from us in the north-east. The velocity
pattern of the gas finally provides direct evidence that the faint linear
feature seen in images of the star is a circumstellar disk in Keplerian
rotation. From modelling the spatial distribution of the Na I line profiles we
determine the effective dynamical mass to be 1.40 +/- 0.05 M_sun, which is
smaller than the stellar mass, 1.75 M_sun. We ascribe this difference to the
gravity opposing radiation pressure in the Na I lines. We argue that this is
consistent with the fact that Na is nearly completely ionised throughout the
disk (Na I/Na < 10^-4). The total column density of sodium gas is N(Na) = 10^15
cm^-2.Comment: 9 pages, including 6 figs (fig.1 in colour). Accepted by ApJ
Abundance Analysis of HE2148-1247, A Star With Extremely Enhanced Neutron Capture Elements
Abundances for 27 elements in the very metal poor dwarf star HE2148-1247 are
presented, including many of the neutron capture elements. We establish that
HE2148-1247 is a very highly s-process enhanced star with anomalously high Eu
as well, Eu/H about half Solar, demonstrating the large addition of heavy
nuclei at [Fe/H] = -2.3 dex. Ba and La are enhanced by a somewhat larger factor
and reach the solar abundance, while Pb significantly exceeds it. Ba/Eu is ten
times the solar r-process ratio but much less than that of the s-process,
indicating a substantial r-process addition as well. C and N are also very
highly enhanced. We have found that HE2148-1247 is a radial velocity variable.
The C, N and the s-process element enhancements thus presumably were produced
through mass transfer from a former AGB binary companion. The large enhancement
of heavy r-nuclides also requires an additional source as this is far above any
inventory in the ISM at such low [Fe/H]. We further hypothesize that accretion
onto the white dwarf from the envelope of the star caused accretion induced
collapse of the white dwarf, forming a neutron star, which then produced heavy
r-nuclides and again contaminated its companion. (abridged)Comment: Accepted by the Astrophysical Journal. Companion paper by Qian and
Wasserburg follow
A Study of the Near-Ultraviolet Spectrum of Vega
UV, optical, and near-IR spectra of Vega have been combined to test our
understanding of stellar atmospheric opacities, and to examine the possibility
of constraining chemical abundances from low-resolution UV fluxes. We have
carried out a detailed analysis assuming Local Thermodynamic Equilibrium (LTE)
to identify the most important contributors to the UV continuous opacity: H,
H, C I, and Si II. Our analysis also assumes that Vega is spherically
symmetric and its atmosphere is well described with the plane parallel
approximation. Comparing observations and computed fluxes we have been able to
discriminate between two different flux scales that have been proposed, the
IUE-INES and the HST scales, favoring the latter. The effective temperature and
angular diameter derived from the analysis of observed optical and near-UV
spectra are in very good agreement with previous determinations based on
different techniques. The silicon abundance is poorly constrained by the UV
observations of the continuum and strong lines, but the situation is more
favorable for carbon and the abundances inferred from the UV continuum and
optical absorption lines are in good agreement. Some spectral intervals in the
UV spectrum of Vega that the calculations do not reproduce well are likely
affected by deviations from LTE, but we conclude that our understanding of UV
atmospheric opacities is fairly complete for early A-type stars.Comment: 13 pages, 9 figures, to be published in Ap
Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared
We have assembled new Spitzer Space Telescope Infrared Array Camera
observations of the mysterious binary star Epsilon Aurigae, along with archival
far-ultraviolet to mid-infrared data, to form an unprecedented spectral energy
distribution spanning three orders of magnitude in wavelength from 0.1 microns
to 100 microns. The observed spectral energy distribution can be reproduced
using a three component model consisting of a 2.2+0.9/-0.8 Msun F type
post-asymptotic giant branch star, and a 5.9+/-0.8 Msun B5+/-1 type main
sequence star that is surrounded by a geometrically thick, but partially
transparent, disk of gas and dust. At the nominal HIPPARCOS parallax distance
of 625 pc, the model normalization yields a radius of 135+/-5 Rsun for the F
star, consistent with published interferometric observations. The dusty disk is
constrained to be viewed at an inclination of i > 87 deg, and has effective
temperature of 550+/-50 K with an outer radius of 3.8 AU and a thickness of
0.95 AU. The dust content of the disk must be largely confined to grains larger
than ~10 microns in order to produce the observed gray optical-infrared
eclipses and the lack of broad dust emission features in the archival Spitzer
mid-infrared spectra. The total mass of the disk, even considering a potential
gaseous contribution in addition to the dust that produces the observed
infrared excess, is << 1 Msun. We discuss evolutionary scenarios for this
system that could lead to the current status of the stellar components and
suggests possibilities for its future evolution, as well as potential
observational tests of our model.Comment: 13 pages, 3 figures. Accepted for publication in The Astrophysical
Journal
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