53 research outputs found
Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
In this paper we develop a calculation code to account for the effects
carried by fast rotation on the observed spectra of early-type stars. Stars are
assumed to be in rigid rotation and the grid of plane-parallel model
atmospheres used to represent the gravitational darkening are calculated by
means of a non-LTE approach. Attention is paid on the relation between the
apparent and parent non-rotating counterpart stellar fundamental parameters and
apparent and true vsini parameters as a function of the rotation rate
Omega/Omega_c, stellar mass and inclination angle. It is shown that omission of
gravitational darkening in the analysis of chemical abundances of CNO elements
can produce systematic overestimation or underestimation, depending on the
lines used, rotational rate and inclination angle. The proximity of Be stars to
the critical rotation is re-discussed by correcting not only the vsini of 130
Be stars, but also their effective temperature and gravity to account for
stellar rotationally induced geometrical distortion and for the concomitant
gravitational darkening effect. We concluded that the increase of the vsini
estimate is accompanied by an even higher value of the stellar equatorial
critical velocity, so that the most probable average rate of angular velocity
of Be stars attains Omega/Omega_c ~ 0.88.Comment: 20 pages, 16 figures. Submitted for publication in A&
Optical spectroscopy of the large Kuiper Belt objects 136472 (2005 FY9) and 136108 (2003 EL61).
We present high signal precision optical reflectance spectra of the large Kuiper Belt objects 2005 FY9 and 2003 EL61. The spectrum of 2005 FY9 exhibits strong CH4 ice bands. A comparison between the spectrum and a Hapke model indicates that the CH4 bands are shifted 3.25 ± 2.25 Å relative to pure CH4 ice, suggesting the presence of another ice component on the surface of 2005 FY9, possibly N2 ice, CO ice, or Ar. The spectrum of 2003 EL61 is remarkably featureless. There is a hint of an O2 ice band at 5773 Å; however, this feature needs to be confirmed by future spectroscopic observations of 2003 EL61 with a higher continuum signal precision sufficient to detect a second, weaker O2 ice band at 6275 Å. [on SciFinder(R)
Keck/HIRES Spectroscopy of Four Candidate Solar Twins
We use high S/N, high-resolution Keck/HIRES spectroscopy of 4 solar twin
candidates (HIP 71813, 76114, 77718, 78399) from our Hipparcos-based CaII H & K
survey to carry out parameter and abundance analyses of these objects. Our
spectroscopic Teff estimates are some 100 K hotter than the photometric scale
of the recent Geneva-Copenhagen survey; several lines of evidence suggest the
photometric temperatures are too cool at solar . At the same time,
our abundances for the 3 solar twin candidates included in the
Geneva-Copenhagen survey are in outstanding agreement with the photometric
metallicities; there is no sign of the anomalously low photometric
metallicities derived for some late-G UMa group and Hyades dwarfs. A first
radial velocity determination is made for HIP 78399, and UVW kinematics derived
for all stars. HIP 71813 appears to be a kinematic member of the Wolf 630
moving group (a structure apparently reidentified in a recent analysis of
late-type Hipparcos stars), but its metallicity is 0.1 dex higher than the most
recent estimate of this group's metallicity. While certainly ``solar-type''
stars, HIP 76114 and 77718 are a few percent less massive, significantly older,
and metal-poor compared to the Sun; they are neither good solar twin candidates
nor solar analogs providing a look at the Sun at some other point in its
evolution. HIP 71813 appears to be an excellent solar analog of age 8 Gyr. Our
results for HIP 78399 suggest the promise of this star as a solar twin may be
equivalent to the ``closest ever solar twin'' HR 6060; follow up study of this
star is encouraged.Comment: Accepted for publication in The Astronomical Journal (November 2005
volume
Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis
CU Vir and V901 Ori belong among these few magnetic chemically peculiar stars
whose rotation periods vary on timescales of decades. We aim to study the
stability of the periods in CU Vir and V901 Ori using all accessible
observational data containing phase information. We collected all available
relevant archived observations supplemented with our new measurements of these
stars and analysed the period variations of the stars using a novel method that
allows for the combination of data of diverse sorts. We found that the shapes
of their phase curves were constant, while the periods were changing. Both
stars exhibit alternating intervals of rotational braking and acceleration. The
rotation period of CU Vir was gradually shortening until the year 1968, when it
reached its local minimum of 0.52067198 d. The period then started increasing,
reaching its local maximum of 0.5207163 d in the year 2005. Since that time the
rotation has begun to accelerate again. We also found much smaller period
changes in CU Vir on a timescale of several years. The rotation period of V901
Ori was increasing for the past quarter-century, reaching a maximum of 1.538771
d in the year 2003, when the rotation period began to decrease. A theoretically
unexpected alternating variability of rotation periods in these stars would
remove the spin-down time paradox and brings a new insight into structure and
evolution of magnetic upper-main-sequence stars.Comment: 5 pages, 3 figure
Observations of Comet 9P/Tempel 1 with the Keck 1 HIRES Instrument During Deep Impact
We report high-spectral resolution observations of comet 9P/Tempel 1 before,
during and after the impact on 4 July 2005 UT of the Deep Impact spacecraft
with the comet. These observations were obtained with the HIRES instrument on
Keck 1. We observed brightening of both the dust and gas, but at different
rates. We report the behavior of OH, NH, CN, C, CH, NH and C
gas. From our observations, we determined a CN outflow velocity of at least
0.51 km sec. The dust color did not change substantially. To date, we
see no new species in our spectra, nor do we see any evidence of prompt
emission. From our observations, the interior material released by the impact
looks the same as the material released from the surface by ambient cometary
activity. However, further processing of the data may uncover subtle
differences in the material that is released as well as the time evolution of
this material.Comment: 22 pages, 5 figures (1 color, landscape). Accepted for publication in
Icaru
A Study of the B-V Colour Temperature Relation
We attempt to construct a B-V colour temperature relation for stars in the
least model dependent way employing the best modern data. The fit we obtained
with the form Teff = Teff((B-V)0,[Fe/H],log g) is well constrained and a number
of tests show the consistency of the procedures for the fit. Our relation
covers from F0 to K5 stars with metallicity [Fe/H] = -1.5 to +0.3 for both
dwarfs and giants. The residual of the fit is 66 K, which is consistent with
what are expected from the quality of the present data. Metallicity and surface
gravity effects are well separated from the colour dependence. Dwarfs and
giants match well in a single family of fit, differing only in log g. The fit
also detects the Galactic extinction correction for nearby stars with the
amount E(B-V) = 0.26 +/-0.03 mag/kpc. Taking the newly obtained relation as a
reference we examine a number of B-V colour temperature relations and
atmosphere models available in the literature. We show the presence of a
systematic error in the colour temperature relation from synthetic calculations
of model atmospheres; the systematic error across K0 to K5 dwarfs is 0.04-0.05
mag in B-V, which means 0.25-0.3 mag in Mv for the K star range. We also argue
for the error in the temperature scale used in currently popular stellar
population synthesis models; synthetic colours from these models are somewhat
too blue for aged elliptical galaxies. We derive the colour index of the sun
(B-V)sun = 0.627 +/-0.018, and discuss that redder colours (e.g., 0.66-0.67)
often quoted in the literature are incompatible with the colour-temperature
relation.Comment: AASLaTeX (aaspp4.sty),36 pages (13 figures included), submitted to
Astronomical Journal, replaced (typo in author name
Chemical compositions of Four B-type Supergiants in the SMC Wing
High-resolution UCLES/AAT spectra of four B-type supergiants in the SMC South
East Wing have been analysed using non-LTE model atmosphere techniques to
determine their atmospheric parameters and chemical compositions. The principle
aim of this analysis was to determine whether the very low metal abundances
(1.1 dex compared with Galactic value) previously found in the Magellanic
Inter Cloud region (ICR) were also present in SMC Wing. The chemical
compositions of the four targets are similar to those found in other SMC
objects and appear to be incompatible with those deduced previously for the
ICR. Given the close proximity of the Wing to the ICR, this is difficult to
understand and some possible explanations are briefly discussed.Comment: 11 pages, 2 figues, A&A accepte
The Discovery of a Planetary Companion to 16 Cygni B
High precision radial velocity observations of the solar-type star 16 Cygni B
taken at McDonald Observatory and at Lick Observatory, have each independently
discovered periodic radial-velocity variations indicating the presence of a
Jovian-mass companion to this star. The orbital fit to the combined data gives
a period of 800.8 days, a velocity amplitude of 43.9 m/s, and an eccentricity
of 0.63. This is the largest eccentricity of any planetary system discovered so
far. Assuming that 16 Cygni B has a mass of 1.0 Msun, this implies a mass for
the companion of 1.5 sin i Jupiter masses. While the mass of this object is
well within the range expected for planets, the large orbital eccentricity
cannot be explained simply by the standard model of growth of planets in a
protostellar disk. It is possible that this object was formed in the normal
manner with a low eccentricity orbit, and has undergone post-formational
orbital evolution, either through the same process which formed the ``massive
eccentric'' planets around 70 Virginis and HD114762, or by gravitational
interactions with the companion star 16 Cygni A. It is also possible that the
object is an extremely low mass brown dwarf, formed through fragmentation of
the collapsing protostar. We explore a possible connection between stellar
photospheric Li depletion, pre-main sequence stellar rotation, the presence of
a massive proto-planetary disk, and the formation of a planetary companion.Comment: 18 pages, 3 PostScript figures, Latex, uses aaspp4 macros, submitted
to Astrophysical Journa
On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking into account
the effects induced by the fast rotation on their fundamental parameters. All
program stars were observed in the BCD spectrophotometric system in order to
minimize the perturbations produced by the circumstellar environment on the
spectral photospheric signatures. This is one of the first attempts at
determining stellar masses and ages by simultaneously using model atmospheres
and evolutionary tracks, both calculated for rotating objects. The stellar ages
() normalized to the respective inferred time that each rotating star can
spend in the main sequence phase () reveal a mass-dependent
trend. This trend shows that: a) there are Be stars spread over the whole
interval 0 \la \tau/\tau\_{\rm MS} \la 1 of the main sequence evolutionary
phase; b) the distribution of points in the () diagram indicates that in massive stars (M \ga
12M\_{\odot}) the Be phenomenon is present at smaller
age ratios than for less massive stars (M \la 12M\_{\odot}). This
distribution can be due to: ) higher mass-loss rates in massive objets,
which can act to reduce the surface fast rotation; ) circulation time
scales to transport angular momentum from the core to the surface, which are
longer the lower the stellar mass.Comment: 18 pages, 6 figures, A&A, in pres
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