155 research outputs found
Abundances in Przybylski's star
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74704/1/j.1365-8711.2000.03578.x.pd
The HII Region KR 140: Spontaneous Formation of a High Mass Star
We have used a multiwavelength data set from the Canadian Galactic Plane
Survey (CGPS) to study the Galactic HII region KR 140, both on the scale of the
nebula itself and in the context of the star forming activity in the nearby
W3/W4/W5 complex of molecular clouds and HII regions. From both radio and
infrared data we have found a covering factor of about 0.5 for KR 140 and we
interpret the nebula as a bowl-shaped region viewed close to face on.
Extinction measurements place the region on the near side of its parent
molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735,
which is less than a few million years old. CO data show that VES 735 has
disrupted much of the original molecular cloud for which the estimated mass and
density are about 5000 and 100 cm, respectively. KR 140 is
isolated from the nearest star forming activity, in W3. Our data suggest that
KR 140 is an example of spontaneous (i.e., non-triggered) formation of,
unusually, a high mass star.Comment: 46 pages; includes 15 figures; accepted by the Ap
Weak G-band stars on the H-R Diagram: Clues to the origin of Li anomaly
Weak G-band (WGB) stars are a rare class of cool luminous stars that present
a strong depletion in carbon, but also lithium abundance anomalies that have
been little explored in the literature since the first discovery of these
peculiar objects in the early 50's. Here we focus on the Li-rich WGB stars and
report on their evolutionary status. We explore different paths to propose a
tentative explanation for the lithium anomaly. Using archive data, we derive
the fundamental parameters of WGB (Teff, log g, log(L/Lsun)) using Hipparcos
parallaxes and recent temperature scales. From the equivalent widths of Li
resonance line at 6707 {\AA}, we uniformly derive the lithium abundances and
apply when possible NLTE corrections following the procedure described by Lind
et al. (2009). We also compute dedicated stellar evolution models in the mass
range 3.0 to 4.5 Msun, exploring the effects of rotation-induced and
thermohaline mixing. These models are used to locate the WGB stars in the H-R
diagram and to explore the origin of the abundance anomalies. The location of
WGB stars in the H-R diagram shows that these are intermediate mass stars of
masses ranging from 3.0 to 4.5 Msun located at the clump, which implies a
degeneracy of their evolutionary status between subgiant/red giant branch and
core helium burning phases. The atmospheres of a large proportion of WGB stars
(more than 50%) exhibit lithium abundances A(Li) \geq 1.4 dex similar to
Li-rich K giants. The position of WGB stars along with the Li-rich K giants in
the H-R diagram however indicates that both are well separated groups. The
combined and tentatively consistent analysis of the abundance pattern for
lithium, carbon and nitrogen of WGB stars seems to indicate that carbon
underabundance could be decorrelated from the lithium and nitrogen
overabundances.Comment: 13 pages, 3 figures, Accepted for publication in Astronomy and
Astrophysic
An evolutionary catalogue of Galactic post-AGB and related objects
With the ongoing AKARI infrared sky survey, of much greater sensitivity than
IRAS, a wealth of post-AGB objects may be discovered. It is thus time to
organize our present knowledge of known post-AGB stars in th galaxy with a view
to using it to search for new post-AGB objects among AKARI sources. We searched
the literature available on the NASA Astrophysics Data System up to 1 October
2006, and defined criteria for classifying sources into three categories: "very
likely", "possible" and "disqualified" post-AGB objects. The category of "very
likely" post-AGB objects is made up of several classes. We have created an
evolutionary, on-line catalogue of Galactic post-AGB objects, to be referred to
as "The Torun catalogue of Galactic post-AGB and related objects". The present
version of the catalogue contains 326 "very likely", 107 "possible" and 64
"disqualified" objects. For the very likely post-AGB objects, the catalogue
gives the available optical and infrared photometry, infrared spectroscopy and
spectral types, and links to finding charts and bibliography.Comment: 9 pages, 6 figures, accepted for publication in A&
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
Aims. We aim to provide observational constraints on diffusion models that
predict peculiar chemical abundances in the atmospheres of Am stars. We also
intend to check if chemical peculiarities and slow rotation can be explained by
the presence of a weak magnetic field.
Methods. We have obtained high resolution, high signal-to-noise ratio spectra
of eight previously-classified Am stars, two normal A-type stars and one Blue
Straggler, considered to be members of the Praesepe cluster. For all of these
stars we have determined fundamental parameters and photospheric abundances for
a large number of chemical elements, with a higher precision than was ever
obtained before for this cluster. For seven of these stars we also obtained
spectra in circular polarization and applied the LSD technique to constrain the
longitudinal magnetic field.
Results. No magnetic field was detected in any of the analysed stars. HD
73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to
have the abundances of a normal A-type star. Am classification is not confirmed
for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry
that is in good agreement with the observations. There is a generally good
agreement between abundance predictions of diffusion models and values that we
have obtained for the remaining Am stars. However, the observed Na and S
abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.
Li appears to be overabundant in three stars of our sample.Comment: Accepted for publication on A&
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
Chemical composition of A--F type post-AGB candidates
An abundance analysis has been conducted for a sample of nine post-AGB
candidate stars; eight of them have not been explored before. We find four very
promising objects like HD 105262, HD 53300 and CpD among them. We
find strong evidence of dust-gas separation through selective depletion of
refractive elements in HD 105262. The same effect is also observed in HD 53300,
CpD and HD 114855 although abundance peculiarities are relatively
smaller for the last two stars. We find strong enrichment of nitrogen for HD
725, HD 842, HD 1457, HD 9233 and HD 61227 but no further evidence to support
their post-AGB nature. We have compared the observed [N/C] ratios of these
stars with the predictions of evolutionary models which include the rotation
induced mixing.Comment: Accepted in MNRAS, 18 pages, 12 figure
Astronomical Spectroscopy
Spectroscopy is one of the most important tools that an astronomer has for
studying the universe. This chapter begins by discussing the basics, including
the different types of optical spectrographs, with extension to the ultraviolet
and the near-infrared. Emphasis is given to the fundamentals of how
spectrographs are used, and the trade-offs involved in designing an
observational experiment. It then covers observing and reduction techniques,
noting that some of the standard practices of flat-fielding often actually
degrade the quality of the data rather than improve it. Although the focus is
on point sources, spatially resolved spectroscopy of extended sources is also
briefly discussed. Discussion of differential extinction, the impact of
crowding, multi-object techniques, optimal extractions, flat-fielding
considerations, and determining radial velocities and velocity dispersions
provide the spectroscopist with the fundamentals needed to obtain the best
data. Finally the chapter combines the previous material by providing some
examples of real-life observing experiences with several typical instruments.Comment: An abridged version of a chapter to appear in Planets, Stars and
Stellar Systems, to be published in 2011 by Springer. Slightly revise
Solar BaII 4554 A line as Doppler diagnostics: NLTE analysis in 3D hydrodynamical model
The aim of this paper is to analyse the validity of the Dopplergram and
lambda-meter techniques for the Doppler diagnostics of solar photospheric
velocities using the BaII 4554 A line. Both techniques are evaluated by means
of NLTE radiative transfer calculations of the BaII 4554 A line in a
three-dimensional hydrodynamical model of solar convection. We consider the
cases of spatially unsmeared profiles and the profiles smeared to the
resolution of ground-based observations. We find that: (i)
Speckle-reconstructed Dopplergram velocities reproduce the ``true'' velocities
well at heights around 300 km, except for intergranular lanes with strong
downflows where the velocity can be overestimated. (ii) The lambda-meter
velocities give a good representation of the ``true'' velocities through the
whole photosphere, both under the original and reduced spatial resolutions. The
velocities derived from the inner wing of smeared BaII 4554 A line profiles are
more reliable than those for the outer wing. Only under high spatial resolution
does the inner wing velocities calculated in intergranular regions give an
underestimate (or even a sign reversal) compared with the model velocities.
(iii) NLTE effects should be taken into account in modelling the BaII 4554 A
line profiles. Such effects are more pronounced in intergranular regions. Our
analysis supports the opinion that the Dopplergram technique applied to the
BaII 4554 A line is a valuable tool for the Doppler diagnostics of the middle
photosphere around 300 km. The \lambda-meter technique applied to this line
gives us a good opportunity to ``trace'' the non-thermal motions along the
whole photosphere up to the temperature minimum and lower chromosphere.Comment: accepted by Astronomy and Astrophysic
Abundance anomalies in hot horizontal branch stars of the galactic globular cluster NGC 1904
We present abundance measurements, based on high-resolution optical
spectroscopic data obtained with the Ultraviolet and Visual Echelle
Spectrograph mounted on Kueyen (Very Large Telescope UT2), for ten stars in the
extended blue horizontal branch of the Galactic globular cluster NGC 1904
(M79). In agreement with previous findings for other clusters, we obtain normal
abundances for stars cooler than Teff ~ 11,000 K and largely anomalous
abundances for hotter stars: large He depletions, and overabundances of Fe, Ti,
Cr, P and Mn. The abundances of Mg, Si and Ca are roughly normal, in the hot
stars as well as in the cooler ones. This abundance pattern can be attributed
to the onset of diffusion and to radiation pressure in the stable atmospheres
of hot horizontal branch stars.Comment: 12 pages, 7 tables, 11 figures, Astronomy & Astrophysics in pres
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