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Star HD (star + hd)

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Physics and Astronomy	100%

Selected Abstracts

HD 209621: abundances of neutron-capture elements,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2010
Aruna Goswami
ABSTRACT High-resolution spectra obtained from the Subaru Telescope High Dispersion Spectrograph have been used to update the stellar atmospheric parameters and metallicity of the star HD 209621. We have derived a metallicity of [Fe/H]=,1.93 for this star, and have found a large enhancement of carbon and of heavy elements, with respect to iron. Updates on the elemental abundances of four s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. The stellar atmospheric parameters, the effective temperature, Teff, and the surface gravity, log g (4500 K, 2.0), are determined from local thermodynamic equilibrium analysis using model atmospheres. Estimated [Ba/Eu]=+0.35, places the star in the group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD 209621 is characteristic of CH stars; notably, the second-peak s-process elements are more enhanced than the first-peak s-process elements. HD 209621 is also found to show a large enhancement of the third-peak s-process element lead (Pb) with [Pb/Fe]=+1.88. The relative contributions of the two neutron-capture processes, r and s, to the observed abundances are examined using a parametric model-based analysis, which hints that the neutron-capture elements in HD 209621 primarily originate in s-process. [source]

The determination of the rotation period and magnetic field geometry of the strongly magnetic roAp star HD 154708,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
S. Hubrig
ABSTRACT We obtained 13 spectropolarimetric observations of the strongly magnetic rapidly oscillating Ap star HD 154708 over 3 months with the multimode instrument FORS 1, installed at the 8-m Kueyen telescope of the Very Large Telescope. These observations have been used for the determination of the rotation period of P= 5.3666 ± 0.0007 d. Using stellar fundamental parameters and the longitudinal magnetic field phase curve, we briefly discuss the magnetic field geometry. The star is observed nearly pole-on and the magnetic field geometry can be described by a centred dipole with a surface polar magnetic field strength Bd between 26.1 and 28.8 kG and an inclination of the magnetic axis to the rotation axis in the range to . [source]

Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines,

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
L. M. Freyhammer
ABSTRACT Chemically peculiar A stars (Ap) are extreme examples of the interaction of atomic element diffusion processes with magnetic fields in stellar atmospheres. The rapidly oscillating Ap stars provide a means for studying these processes in three dimensions and are at the same time important for studying the pulsation excitation mechanism in A stars. As part of the first comprehensive, uniform, high-resolution spectroscopic survey of Ap stars, which we are conducting in the Southern hemisphere with the Michigan Spectral Catalogues as the basis of target selection, we report here the discovery of 17 new magnetic Ap stars having spectroscopically resolved Zeeman components from which we derive magnetic field moduli in the range 3,30 kG. Among these are (1) the current second strongest known magnetic A star, (2) a double-lined Ap binary with a magnetic component and (3) an A star with particularly peculiar and variable abundances. Polarimetry of these stars is needed to constrain their field geometries and to determine their rotation periods. We have also obtained an additional measurement of the magnetic field of the Ap star HD 92499. [source]

Evolution of the 3.3-,m emission feature in the Red Rectangle

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003
In-Ok Song
ABSTRACT Medium-resolution infrared spectroscopic observations of the biconical Red Rectangle nebula in the region of the 3.3-,m unidentified infrared (UIR) emission band are reported. The data were recorded at UKIRT using CGS4 and have allowed the peak wavelength, width, profile and intensity of the 3.3-,m feature to be investigated as a function of offset from the central star HD 44179. Analysis of the profile of the feature along the north-western bicone interface shows an evolution from Type 2 to Type 1 in the classification of Tokunaga et al. The 3.3-,m band recorded on-star shows a close fit to a Lorentzian profile, possibly suggesting a single class of carriers. Subtraction of the Lorentzian fit to the on-star spectrum from the spectrum at each offset reveals a new ,3.28'-,m emission feature that grows in intensity relative to the main 3.3-,m band as a function of distance from the central star. The 3.28-,m emission band at large offset appears to correspond well in wavelength and FWHM with an absorption feature seen towards the Galactic Centre. [source]

Ultra-high-resolution observations of circumstellar K i and C2 around the post-AGB star HD 56126

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
I. A. Crawford
We have used the Ultra-High-Resolution Facility (UHRF) at the AAT, operating at a resolution of 0.35 km s,1 (FWHM), to observe K i and C2 absorption lines arising in the circumstellar environment of the post-AGB star HD 56126. We find three narrow circumstellar absorption components in K i, two of which are also present in C2. We attribute this velocity structure to discrete shells resulting from multiple mass-loss events from the star. The very high spectral resolution has enabled us to resolve the intrinsic linewidths of these narrow lines for the first time, and we obtain velocity dispersions (b -values) of 0.2,0.3 km s,1 for the K i components, and 0.54±0.03 km s,1 for the strongest (and best defined) C2 component. These correspond to rigorous kinetic temperature upper limits of 211 K for K i and 420 K for C2, although the b -value ratio implies that these two species do not co-exist spatially. The observed degree of rotational excitation of C2 implies low kinetic temperatures (Tk,10 K) and high densities (n,106 to 107 cm,3) within the shell responsible for the main C2 component. Given this low temperature, the line profiles then imply either mildly supersonic turbulence or an unresolved velocity gradient through the shell. [source]

Measurements of mean longitudinal magnetic fields in the Of?p stars HD 108 and HD 191612,

ASTRONOMISCHE NACHRICHTEN, Issue 8 2010
S. Hubrig
Abstract Using polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope, we detect a longitudinal magnetic field ,Bz, = ,168±35 G in the Of?p star HD 108. This result is in agreement with the longitudinal magnetic field measurement of the order of ,150 G recently reported by the MiMeS team. The measurement of the longitudinal magnetic field in the Of?p star HD 191612 results in ,Bz, = +450±153 G. The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field ,Bz, = ,220±38 G, indicating a change of polarity over ,100 days. Further, we report the detection of distinct Zeeman features in the narrow Ca II and Na I doublet lines for both Of?p stars, hinting at the possible presence of material around these stars. The origin of these features is not yet clear and more work is needed to investigate how magnetic fields interact with stellar wind dynamics (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

HD 1: The number-one star in the sky,

ASTRONOMISCHE NACHRICHTEN, Issue 4 2010
K.G. Strassmeier
Abstract We present the first ever study of the bright star HD 1. The star was chosen arbitrarily just because of its outstanding Henry Draper number. Surprisingly, almost nothing is known about this bright 7.m4 star. Our observations were performed as part of the commissioning of the robotic telescope facility STELLA and its fiber-fed high-resolution optical echelle spectrograph SES in the years 2007,2010. We found long-term radial velocity variations with a full amplitude of 9 km s,1 with an average velocity of ,29.8 km s,1 and suggest the star to be a hitherto unknown single-lined spectroscopic binary. A preliminary orbit with a period of 6.2 years (2279±69 days) and an eccentricity of 0.50±0.01 is given. Its rms uncertainty is just 73 m s,1. HD 1 appears to be a G9-K0 giant of luminosity class IIIa with Teff = 4850±100 K, logg = 2.0±0.2, L , 155 L,, a mass of 3.0±0.3 M,, a radius of 17.7 R,, and an age of ,350 Myr. A relative abundance analysis led to a metallicity of [Fe/H] = ,0.12 ± 0.09. The , -element silicon may indicate an overabundance of +0.13 though. The low strengths of some s-process lines and a lower limit for the 12C/13C isotope ratio of ,16 indicate that HD 1 is on the first ascend of the RGB. The absorption spectral lines appear rotationally broadened with a v sin i of 5.5±1.2 km s,1 but no chromospheric activity is evident. We also present photometric monitoring BV (RI)C data taken in parallel with STELLA. The star is likely a small-amplitude (<10 mmag) photometric variable although no periodicity was found (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Magnetic survey of emission line B-type stars with FORS 1 at the VLT,

ASTRONOMISCHE NACHRICHTEN, Issue 7 2009
S. Hubrig
Abstract We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star , Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ,1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, , Cen, o Aqr, and , Tuc. The strongest longitudinal magnetic field, ,Bz, = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, , Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star , Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field ,Bz, = ,102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

New magnetic field measurements of , Cephei stars and slowly pulsating B stars,

ASTRONOMISCHE NACHRICHTEN, Issue 4 2009
S. Hubrig
Abstract We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate , Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three , Cephei stars and two stars suspected to be , Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3, has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of , Cephei stars have detected magnetic fields: Out of 13 , Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected , Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i -values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

High-eccentricity planets from the Anglo-Australian Planet Search

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006
Hugh R. A. Jones
ABSTRACT We report Doppler measurements of the stars HD 187085 and HD 20782 which indicate two high eccentricity low-mass companions to the stars. We find HD 187085 has a Jupiter-mass companion with a ,1000-d orbit. Our formal ,best-fitting' solution suggests an eccentricity of 0.47, however, it does not sample the periastron passage of the companion and we find that orbital solutions with eccentricities between 0.1 and 0.8 give only slightly poorer fits (based on rms and ,2,) and are thus plausible. Observations made during periastron passage in 2007 June should allow for the reliable determination of the orbital eccentricity for the companion to HD 187085. Our data set for HD 20782 does sample periastron and so the orbit for its companion can be more reliably determined. We find the companion to HD 20782 has M sin i= 1.77 ± 0.22 MJup, an orbital period of 595.86 ± 0.03 d and an orbit with an eccentricity of 0.92 ± 0.03. The detection of such high-eccentricity (and relatively low-velocity amplitude) exoplanets appears to be facilitated by the long-term precision of the Anglo-Australian Planet Search. Looking at exoplanet detections as a whole, we find that those with higher eccentricity seem to have relatively higher velocity amplitudes indicating higher mass planets and/or an observational bias against the detection of high-eccentricity systems. [source]