Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The Most Metal-poor Stars. IV. The Two Populations with [Fe/H] <~ -3.0
We discuss the carbon-normal and carbon-rich populations of Galactichalo stars having [Fe/H] lsim -3.0, utilizing chemical abundancesfrom high-resolution, high signal-to-noise model-atmosphere analyses.The C-rich population represents ~28% of stars below [Fe/H] =-3.1, with the present C-rich sample comprising 16 CEMP-no stars,and two others with [Fe/H] ~ -5.5 and uncertain classification.The population is O-rich ([O/Fe] gsim +1.5); the light elements Na, Mg,and Al are enhanced relative to Fe in half the sample; and for Z > 20(Ca) there is little evidence for enhancements relative to solar values.These results are best explained in terms of the admixing and processingof material from H-burning and He-burning regions as achieved bynucleosynthesis in zero-heavy-element models in the literature of"mixing and fallback" supernovae (SNe); of rotating, massive, andintermediate-mass stars; and of Type II SNe with relativistic jets. Theavailable (limited) radial velocities offer little support for theC-rich stars with [Fe/H] < -3.1 being binary. More data arerequired before one could conclude that binarity is key to anunderstanding of this population. We suggest that the C-rich andC-normal populations result from two different gas cooling channels inthe very early universe of material that formed the progenitors of thetwo populations. The first was cooling by fine-structure linetransitions of C II and O I (to form the C-rich population); the second,while not well defined (perhaps dust-induced cooling?), led to theC-normal group. In this scenario, the C-rich population contains theoldest stars currently observed.
| The Most Metal-poor Stars. II. Chemical Abundances of 190 Metal-poor Stars Including 10 New Stars with [Fe/H] <= -3.5
We present a homogeneous chemical abundance analysis of 16 elements in190 metal-poor Galactic halo stars (38 program and 152 literatureobjects). The sample includes 171 stars with [Fe/H] <= -2.5, ofwhich 86 are extremely metal poor, [Fe/H] <= -3.0. Our programstars include 10 new objects with [Fe/H] <= -3.5. We identify asample of "normal" metal-poor stars and measure the trends between[X/Fe] and [Fe/H], as well as the dispersion about the mean trend forthis sample. Using this mean trend, we identify objects that arechemically peculiar relative to "normal" stars at the same metallicity.These chemically unusual stars include CEMP-no objects, one star withhigh [Si/Fe], another with high [Ba/Sr], and one with unusually low[X/Fe] for all elements heavier than Na. The Sr and Ba abundancesindicate that there may be two nucleosynthetic processes at lowestmetallicity that are distinct from the main r-process. Finally, for manyelements, we find a significant trend between [X/Fe] versus Teff, which likely reflects non-LTE and/or three-dimensionaleffects. Such trends demonstrate that care must be exercised when usingabundance measurements in metal-poor stars to constrain chemicalevolution and/or nucleosynthesis predictions.This paper includes data gathered with the 6.5 m MagellanTelescopes located at Las Campanas Observatory, Chile.Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile (proposal281.D-5015).
| High-resolution Spectroscopy of Extremely Metal-poor Stars from SDSS/SEGUE. I. Atmospheric Parameters and Chemical Compositions
Chemical compositions are determined based on high-resolutionspectroscopy for 137 candidate extremely metal-poor (EMP) stars selectedfrom the Sloan Digital Sky Survey (SDSS) and its first stellarextension, the Sloan Extension for Galactic Understanding andExploration (SEGUE). High-resolution spectra with moderatesignal-to-noise (S/N) ratios were obtained with the High DispersionSpectrograph of the Subaru Telescope. Most of the sample (approximately80%) are main-sequence turnoff stars, including dwarfs and subgiants.Four cool main-sequence stars, the most metal-deficient such starsknown, are included in the remaining sample. Good agreement is foundbetween effective temperatures estimated by the SEGUE stellar parameterpipeline, based on the SDSS/SEGUE medium-resolution spectra, and thoseestimated from the broadband (V - K)0 and (g -r)0 colors. Our abundance measurements reveal that 70 starsin our sample have [Fe/H] < -3, adding a significant number ofEMP stars to the currently known sample. Our analyses determine theabundances of eight elements (C, Na, Mg, Ca, Ti, Cr, Sr, and Ba) inaddition to Fe. The fraction of carbon-enhanced metal-poor stars ([C/Fe]> +0.7) among the 25 giants in our sample is as high as 36%, whileonly a lower limit on the fraction (9%) is estimated for turnoff stars.This paper is the first of a series of papers based on theseobservational results. The following papers in this series will discussthe higher-resolution and higher-S/N observations of a subset of thissample, the metallicity distribution function, binarity, andcorrelations between the chemical composition and kinematics ofextremely metal-poor stars.
| A catalogue of Paschen-line profiles in standard stars
We have assembled an atlas of line profiles of the Paschen Delta(Pδ) line at 10 049 Å for the use of stellar modelling. Fora few stars we have substituted the Paschen Gamma (Pγ) line at 10938 Å because the Pδ line blends with other features. Mostof the targets are standard stars of spectral types from B to M. A fewmetal-poor stars have been included. For many of the stars we have alsoobserved the Hydrogen Alpha (Hα) line so as to compare theprofiles of lines originating from the meta-stable n = 2 level withlines originating from the n = 3 level. The greatest difference in lineprofile is found for high luminosity and cool stars where the departuresfrom local thermodynamic equilibrium (LTE) in the population of the n =2 level is expected to be the greatest. For a few stars, sample lineprofiles have been calculated in the LTE approximation to demonstratethe usefulness of the tabulated and displayed catalogue.The reduced observed spectra are only available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/547/A62
| E-BOSS: an Extensive stellar BOw Shock Survey. I. Methods and first catalogue
Context. Bow shocks are produced by many astrophysical objects whereshock waves are present. Stellar bow shocks, generated by runaway stars,have been previously detected in small numbers and well-studied. Alongwith progress in model development and improvements in observinginstruments, our knowledge of the emission produced by these objects andits origin can now be more clearly understood. Aims: We produce astellar bow-shock catalogue by applying uniform search criteria and asystematic search process. This catalogue is a starting point forstatistical studies, to help us address fundamental questions such as,for instance, the conditions under wich a stellar bow shock isdetectable. Methods: By using the newest infrared data releases,we carried out a search for bow shocks produced by early-type runawaystars. We first explored whether a set of known IRAS bow shockcandidates are visible in the most recently available IR data, which hasmuch higher resolution and sensitivity. We then carried out a selectionof runaway stars from the latest, large runaway catalogue available. Inthis first release, we focused on OB stars and searched for bow-shapedfeatures in the vicinity of these stars. Results: We provide abow-shock candidate survey that gathers a total of 28 members, which wecall the Extensive stellar BOw Shock Survey (E-BOSS). We derive the mainbow-shock parameters, and present some preliminary statistical resultson the detected objects. Conclusions: Our analysis of the initialsample and the newly detected objects yields a bow-shock detectabilityaround OB stars of ~10 per cent. The detections do not seem to dependparticularly on either stellar mass, age or position. The extension ofthe E-BOSS sample, with upcoming IR data, and by considering, forexample, other spectral types as well, will allow us to perform a moredetailed study of the findings.
| Carbon-enhanced Metal-poor Stars in the Inner and Outer Halo Components of the Milky Way
Carbon-enhanced metal-poor (CEMP) stars in the halo components of theMilky Way are explored, based on accurate determinations of thecarbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities forover 30,000 calibration stars from the Sloan Digital Sky Survey. Usingour present criterion that low-metallicity stars exhibiting [C/Fe]ratios ("carbonicity") in excess of [C/Fe] =+0.7 are considered CEMPstars, the global frequency of CEMP stars in the halo system for [Fe/H]<-1.5 is 8%, for [Fe/H] <-2.0 it is 12%, and for [Fe/H] <-2.5it is 20%. We also confirm a significant increase in the level of carbonenrichment with declining metallicity, growing from lang[C/Fe]rang ~+1.0at [Fe/H] =-1.5 to lang[C/Fe]rang ~+1.7 at [Fe/H] =-2.7. The nature ofthe carbonicity distribution function (CarDF) changes dramatically withincreasing distance above the Galactic plane, |Z|. For |Z| <5 kpc,relatively few CEMP stars are identified. For distances |Z| >5 kpc,the CarDF exhibits a strong tail toward high values, up to [C/Fe] >+3.0. We also find a clear increase in the CEMP frequency with |Z|. Forstars with -2.0 < [Fe/H] <-1.5, the frequency grows from 5% at |Z|~2 kpc to 10% at |Z| ~10 kpc. For stars with [Fe/H] <-2.0, thefrequency grows from 8% at |Z| ~2 kpc to 25% at |Z| ~10 kpc. For starswith -2.0 < [Fe/H] <-1.5, the mean carbonicity is lang[C/Fe]rang~+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for[Fe/H] <-2.0, lang[C/Fe]rang ~+1.5, again roughly independent of |Z|.Based on a statistical separation of the halo components in velocityspace, we find evidence for a significant contrast in the frequency ofCEMP stars between the inner- and outer-halo components—the outerhalo possesses roughly twice the fraction of CEMP stars as the innerhalo. The carbonicity distribution also differs between the inner-haloand outer-halo components—the inner halo has a greater portion ofstars with modest carbon enhancement ([C/Fe] ~+0.5]); the outer halo hasa greater portion of stars with large enhancements ([C/Fe] ~+2.0),although considerable overlap still exists. We interpret these resultsas due to the possible presence of additional astrophysical sources ofcarbon production associated with outer-halo stars, beyond theasymptotic giant-branch source that may dominate for inner-halo stars,with implications for the progenitors of these populations.
| Beryllium and Alpha-element Abundances in a Large Sample of Metal-poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects ofastronomy including stellar structure, Galactic evolution, andcosmology. We have made observations of Be in 117 metal-poor starsranging in metallicity from [Fe/H] = -0.5 to -3.5 with KeckI/HIRES. Our spectra are high resolution (~42,000) and high signal tonoise (the median is 106 per pixel). We have determined the stellarparameters spectroscopically from lines of Fe I, Fe II, Ti I, and Ti II.The abundances of Be and O were derived by spectrum synthesistechniques, while abundances of Fe, Ti, and Mg were found from manyspectral line measurements. There is a linear relationship between[Fe/H] and A(Be) with a slope of +0.88 ± 0.03 over three ordersof magnitude in [Fe/H]. We find that Be is enhanced relative to Fe;[Be/Fe] is +0.40 near [Fe/H] ~-3.3 and drops to 0.0 near [Fe/H]~-1.7. For the relationship between A(Be) and [O/H], we find agradual change in slope from 0.69 ± 0.13 for the Be-poor/O-poorstars to 1.13 ± 0.10 for the Be-rich/O-rich stars. Inasmuch asthe relationship between [Fe/H] and [O/H] seems robustly linear (slope =+0.75 ± 0.03), we conclude that the slope change in Be versus Ois due to the Be abundance. Much of the Be would have been formed in thevicinity of Type II supernova (SN II) in the early history of the Galaxyand by Galactic cosmic-ray (GCR) spallation in the later eras. AlthoughBe is a by-product of CNO, we have used Ti and Mg abundances asalpha-element surrogates for O in part because O abundances are rathersensitive to both stellar temperature and surface gravity. We find thatA(Be) tracks [Ti/H] very well with a slope of 1.00 ± 0.04. Italso tracks [Mg/H] very well with a slope of 0.88 ± 0.03. We havekinematic information on 114 stars in our sample and they divide equallyinto dissipative and accretive stars. Almost the full range of [Fe/H]and [O/H] is covered in each group. There are distinct differences inthe relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for thedissipative and the accretive stars. It is likely that the formation ofBe in the accretive stars was primarily in the vicinity of SN II, whilethe Be in the dissipative stars was primarily formed by GCR spallation.We find that Be is not as good a cosmochronometer as Fe. We have found aspread in A(Be) that is valid at the 4? level between [O/H] =-0.5 and -1.0, which corresponds to -0.9 and-1.6 in [Fe/H].
| A carbon-enhanced metal-poor damped Lyα system: probing gas from Population III nucleosynthesis?
We present high-resolution observations of an extremely metal-poordamped Lyα system (DLA), at zabs= 2.340 0972 in thespectrum of the QSO J0035-0918, exhibiting an abundance patternconsistent with model predictions for the supernova yields of PopulationIII stars. Specifically, this DLA has [Fe/H]≃-3, shows a clear‘odd-even’ effect, and is C-rich with [C/Fe]=+1.53, a factorof ˜20 greater than reported in any other DLA. In analogy to thecarbon-enhanced metal-poor stars in the Galactic halo (with [C/Fe] >+1.0), this is the first known case of a carbon-enhanced DLA. Wedetermine an upper limit to the mass of 12C,M(12C) ≤ 200 M&sun;, which depends on theunknown gas density n(H); if n(H) > 1 cm-3 (which is quitelikely for this DLA given its low velocity dispersion), thenM(12C) ≤ 2 M&sun;, consistent with pollution byonly a few prior supernovae. We speculate that DLAs such as the onediscovered here may represent the ‘missing link’ between theyields of Population III stars and their later incorporation in theclass of carbon-enhanced metal-poor stars which show no enhancement ofneutron-capture elements (CEMP-no stars). Based on data obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia and NASA, and was made possible by the generous financialsupport of the W. M. Keck Foundation.
| The Stellar Abundances for Galactic Archaeology (SAGA) data base - II. Implications for mixing and nucleosynthesis in extremely metal-poor stars and chemical enrichment of the Galaxy
We discuss the characteristics of known extremely metal-poor (EMP) starsin the Galaxy using the Stellar Abundances for Galactic Archaeology(SAGA) data base. We find the transition of the initial mass function tobe at [Fe/H]˜-2 from the viewpoint of the distribution of carbonabundance and the frequency of carbon-enhanced stars. Analyses ofcarbon-enhanced stars in our sample suggest that nucleosynthesis inasymptotic giant branch (AGB) stars can contribute to carbon enrichmentin a different way depending on whether the metallicity is above orbelow [Fe/H]˜-2.5, which is consistent with the current models ofstellar evolution at low metallicity. For observed EMP stars, we confirmthat some, though not all, observed stars might have undergone at leasttwo types of extra mixing to change their surface abundances. One is thedepletion of lithium abundance during the early phase of the red giantbranch; the other is a decrease of the C/N ratio by one order ofmagnitude during the red giant branch phase. Observed small scatters ofabundances for ?-elements and iron-group elements suggest that thechemical enrichment of our Galaxy takes place in a well-mixedinterstellar medium. The abundance trends of ?-elements are highlycorrelated with each other including ?-enhanced and depletedstars, while the abundances of iron-group elements are subject todifferent slopes relative to the iron abundance. This implies that thesupernova yields of ?-elements are almost independent of mass andmetallicity, while those of iron-group elements have a metallicitydependence or mass dependence on the variable initial mass function. Theoccurrence of the hot-bottom burning for M? 5 M&sun; isconsistent with an initial mass function of the Galaxy peaked at˜10-12 M&sun;, compatible with the statistics ofcarbon-enhanced stars with and without s-process element enhancement andnitrogen-enhanced stars. For s-process elements, we find not only apositive correlation between carbon and s-process element abundances,but also an increasing slope of the abundance ratio between them withincreasing mass number of s-process elements. The dominant site of thes-process is still an open question because none of the known mechanismsfor the s-process is able to account for this observed correlation. Inspite of the evidence of AGB evolution in observed abundances of EMPstars, any evidence of binary mass transfer is elusive by pursuing theeffect of dilution in the convective envelope. We find the dependence ofsulphur and vanadium abundances on the effective temperatures, inaddition to the previously reported trends for silicon, scandium,titanium, chromium and cobalt.
| Chemical Enrichment in the Carbon-enhanced Damped Lyα System by Population III Supernovae
We show that the recently observed elemental abundance pattern of thecarbon-rich metal-poor damped Lyα (DLA) system is in excellentagreement with the nucleosynthesis yields of faint core-collapsesupernovae of primordial stars. The observed abundance pattern is notconsistent with the nucleosynthesis yields of pair-instabilitysupernovae. The DLA abundance pattern is very similar to that ofcarbon-rich extremely metal-poor (EMP) stars, and the contributions fromlow-mass stars and/or binary effects should be very small in DLAs. Thissuggests that chemical enrichment by the first stars in the firstgalaxies is driven by core-collapse supernovae from ~20 to 50 Msun stars and also supports the supernova scenario as theenrichment source of EMP stars in the Milky Way Galaxy.
| Chromospheres in Metal-Poor Stars Evidenced from the He I 10830Å Line
Based on the near-IR spectra of 33 late-type stars in a wide metallicityrange (mainly dwarfs and partly giants) obtained with IRCS+AO188 of theSubaru Telescope, we confirmed that the He I 10830 Å line is seenin absorption in almost all moderately to extremely metal-poor stars ofthick disk and halo population (from [Fe/H] ˜ -0.5 down to [Fe/H]˜ -3.7), the strength of which is almost constant irrespective ofthe metallicity. This is evidence that chromospheric activity at a basallevel persists, even for such old stars, despite that their rotationsare considered to be slowed down and incapable of sustaining a dynamo,suggesting that some kind of chromospheric heating mechanism independentof rotation/magnetism (e.g., acoustic heating) may take place.
| Exploring the [S/Fe] Behavior of Metal-Poor Stars with the Si 1.046?m Lines
In an attempt of clarifying the [S/Fe] behavior with the run of [Fe/H]in the metal-poor regime, which has been a matter of debate, anextensive non-LTE analysis of near-IR Si triplet lines (multiplet 3) at1.046 ?m was carried out for 33 selected halo/disk stars over a widemetallicity range of [Fe/H] ˜ -3.7 to ˜ +0.3, based onspectral data collected with IRCS+AO188 of the Subaru Telescope. Wefound evidence of a considerably large [S/Fe] ratio, amounting to˜ +0.7-0.8 dex at very low metallicity of [Fe/H] ˜ -3, whichmakes marked contrast with other ?-elements (Mg, Si, Ca, Ti)flatly showing a moderately supersolar [?/Fe] of ˜0.3 dex.Meanwhile, a locally-flat tendency of [S/Fe] at ˜+0.3 is seen at-2.5 ? [Fe/H]? -1.5. These results may suggest that the natureof [S/Fe] in metal-poor halo stars is not as simple as has been argued(i.e., neither being globally flat independent of [Fe/H] normonotonically increasing with a decrease in [Fe/H]), but rathercomplicated with a local plateau around [Fe/H] ˜ -2, followed by adiscontinuous jump between the narrow interval of -3 ? [Fe/H]? -2.5.
| [O/Fe] Estimates for Carbon-enhanced Metal-poor Stars from Near-infrared Spectroscopy
We report on oxygen abundances determined from medium-resolutionnear-infrared spectroscopy for a sample of 57 carbon-enhanced metal-poor(CEMP) stars selected from the Hamburg/ESO Survey. The majority of ourprogram stars exhibit oxygen-to-iron ratios in the range +0.5<[O/Fe]<+2.0. The [O/Fe] values for this sample are statisticallycompared to available high-resolution estimates for known CEMP stars aswell as to high-resolution estimates for a set of carbon-normalmetal-poor stars. Carbon, nitrogen, and oxygen abundance patterns for asub-sample of these stars are compared to yield predictions for verymetal-poor asymptotic giant branch (AGB) abundances in the recentliterature. We find that the majority of our sample exhibit patternsthat are consistent with previously studied CEMP stars havings-process-element enhancements and thus have very likely been pollutedby carbon- and oxygen-enhanced material transferred from a metal-poorAGB companion.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| BD+44°493: A Ninth Magnitude Messenger from the Early Universe; Carbon Enhanced and Beryllium Poor
We present a one-dimensional LTE chemical abundance analysis of the verybright (V = 9.1) carbon-enhanced metal-poor (CEMP) star BD+44°493,based on high-resolution, high signal-to-noise spectra obtained withSubaru/HDS. The star is shown to be a subgiant with an extremely lowiron abundance ([Fe/H] = -3.7), while it is rich in C ([C/Fe] = +1.3)and O ([O/Fe] = +1.6). Although astronomers have been searching forextremely metal-poor stars for decades, this is the first star foundwith [Fe/H] <-3.5 and an apparent magnitude V < 12. Based on itslow abundances of neutron-capture elements (e.g., [Ba/Fe] =-0.59),BD+44°493 is classified as a "CEMP-no" star. Its abundance patternimplies that a first-generation faint supernova is the most likelyorigin of its carbon excess, while scenarios related to mass loss fromrapidly rotating massive stars or mass transfer from an asymptotic giantbranch companion star are not favored. From a high-quality spectrum inthe near-UV region, we set an very low upper limit on this star'sberyllium abundance (A(Be) = log(Be/H) + 12 < -2.0), which indicatesthat the decreasing trend of Be abundances with lower [Fe/H] still holdsat [Fe/H] <-3.5. This is the first attempt to measure a Be abundancefor a CEMP star, and demonstrates that high C and O abundances do notnecessarily imply high Be abundances.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars
We report the results for rotational velocities, Vrot sin i,and macroturbulence dispersions, ζRT, for 12 metal-poorfield red giant branch (RGB) stars and 7 metal-poor field red horizontalbranch (RHB) stars. The results are based on Fourier transform analysesof absorption line profiles from high-resolution (R ≈ 120,000),high-S/N (≈215 per pixel; ≈345 per resolution element) spectraobtained with the Gecko spectrograph at the Canada-France-HawaiiTelescope (CFHT). The stars were selected from the authors' previousstudies of 20 RHB and 116 RGB stars, based primarily onlarger-than-average line-broadening values. We find thatζRT values for the metal-poor RGB stars are very similarto those for metal-rich disk giants studied earlier by Gray and hiscollaborators. Six of the RGB stars have small rotational values, lessthan 2.0 km s-1, while five show significantrotation/enhanced line broadening, over 3 km s-1. We confirmthe rapid rotation rate for RHB star HD 195636, found earlier byPreston. This star's rotation is comparable to that of the fastest knownrotating blue horizontal branch (BHB) stars, when allowance is made fordifferences in radii and moments of inertia. The other six RHB starshave somewhat lower rotation but show a trend to higher values at highertemperatures (lower radii). Comparing our results with those for BHBstars from Kinman et al., we find that the fraction of rapidly rotatingRHB stars is somewhat lower than is found among BHB stars. The number ofrapidly rotating RHB stars is also smaller than we would have expectedfrom the observed rotation of the RGB stars. We devise two empiricalmethods to translate our earlier line-broadening results intoVrot sin i for all the RGB and RHB stars they studied.Binning the RGB stars by luminosity, we find that most metal-poor fieldRGB stars show no detectable sign, on average, of rotation, which is notsurprising given the stars' large radii. However, the most luminousstars, with MV <= -1.5, do show net rotation, with meanvalues of 2-4 km s-1, depending on the algorithm employed,and also show signs of radial velocity jitter and mass loss. This"rotation" may in fact prove to be due to other line-broadening effects,such as shock waves or pulsation.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique de France, and the University of Hawaii.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars
We summarize 2007 radial velocity measurements of 91 metal-poor fieldred giants. Excluding binary systems with orbital solutions, ourcoverage averages 13.7 yr per star, with a maximum of 18.0 yr. We reportfour significant findings. (1) Sixteen stars are found to bespectroscopic binaries, and we present orbital solutions for 14 of them.The spectroscopic binary frequency of the metal-poor red giants, with[Fe/H]<=-1.4, for periods less than 6000 days, is 16%+/-4%, which isnot significantly different from that of comparable-metallicity fielddwarfs, 17%+/-2%. The two CH stars in our program, BD -1°2582 and HD135148, are both spectroscopic binaries. (2) Velocity jitter is presentamong about 40% of the giants with MV<=-1.4. The twobest-observed cases, HD 3008 and BD +22°2411, showpseudoperiodicities of 172 and 186 days, longer than any knownlong-period variable in metal-poor globular clusters. Photometricvariability seen in HD 3008 and three other stars showing velocityjitter hints that starspots are the cause. However, the phasing of thevelocity data with the photometry data from Hipparcos is not consistentwith a simple starspot model for HD 3008. We argue against orbitalmotion effects and radial pulsation, so rotational modulation remainsthe best explanation. The implied rotational velocities for HD 3008 andBD +22°2411, both with MV<=-1.4 and R~50Rsolar, exceed 12 km s-1. (3) Including HD 3008and BD +22°2411, we have found signs of significant excess linebroadening in eight of the 17 red giants with MV<=-1.4,which we interpret as rotation. In three cases, BD +30°2034, CD-37°14010, and HD 218732, the rotation is probably induced by tidallocking between axial rotation and the observed orbital motion with astellar companion. But this cannot explain the other five stars in oursample that display signs of significant rotation. This high frequencyof elevated rotational velocities does not appear to be caused bystellar mass transfer or mergers: there are too few main-sequencebinaries with short enough periods. We also note that the lack of anynoticeable increase in mean rotation at the magnitude level of the redgiant branch luminosity function ``bump'' argues against the rapidrotation's being caused by the transport of internal angular momentum tothe surface. Capture of a planetary-mass companion as a red giantexpands in radius could explain the high rotational velocities. (4) Wealso find significant rotation in at least six of the roughly 15 (40%)red horizontal-branch stars in our survey. It is likely that theenhanced rotation seen among a significant fraction of both blue and redhorizontal-branch stars arose when these stars were luminous red giants.Rapid rotation alone therefore appears insufficient cause to populatethe blue side of the horizontal branch. While the largest projectedrotational velocities seen among field blue and red horizontal-branchstars are consistent with their different sizes, neither are consistentwith the large values we find for the largest red giants. This suggeststhat some form of angular momentum loss (and possibly mass loss) hasbeen at work. Also puzzling is the apparent absence of rotation seen infield RR Lyrae variables. Angular momentum transfer and conservation inevolved metal-poor field stars thus pose many interesting questions forthe evolution of low-mass stars.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Distribution and Studies of the Infrared Stellar Population in the Galaxy. VI. The Halo
We present infrared J, H and K observations of 69 local galactic halostars. We produce the two colour (JHK) and the colour magnitude (Kversus (J-K)) diagrams for this stellar sample and compare them with thesame diagrams for the stellar populations in the globular clusters M3,M13, M92 and 47 Tucanae and in the old open cluster M67; we also comparethese diagrams with those for the stellar population in the galacticbulge.
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
| Reddening estimation for halo red giants using UVBY photometry
Updated uvby observations for a larger sample of metal-deficient redgiants are presented and combined with a select sample of data from theliterature transformed to a common system. Using the reddening maps ofBurstein & Heiles (1982), new absolute magnitudes, distances,metallicities, and reddenings are derived for each star. Themetallicities are determined with a revised calibration of them1, (b-y) diagram based upon comparison to a complilation ofrecent spectrsoscopic abundances transformed to a common system. Thephotometric abundances agree very well with the spectroscopic; thedispersion among the residuals for 58 giants is +/- 0.16 dex. Thedereddened indices are used to show that for red giants with (Fe/H) lessthan -1.5, there is a well-defined relation in the c0,(b-y)0 diagram which exhibits only a weak dependence uponmetallicity. Use of the standard relations allows one to obtainreddening estimates for normal halo field giants and globular clustersin the appropriate metallicity range.
| The kinematics of halo red giants
The present 337 radial velocities were obtained with typical accuraciesof 0.7 cm km/sec for 85 metal-poor field red giants, selected from thekinematically unbiased samples of Bond (1980) and Bidelman and MacConnel(1973). The multiply-observed stars suggest the field halo binaryfraction exceeds 10 percent. Using these velocities and those publishedby others, a sample of 174 red giants with Fe/H of not more than -1.5 isobtained. Their mean motion with respect to the local standard of restis -206 + or - 23 km/sec, and the velocity dispersions are sigma (R) of154 + or - 18 km/sec, sigma(theta) of 102 + or - 27 km/sec, andsigma(phi) of 107 + or - 15 km/sec. Using photometrically derivedabsolute magnitudes and published proper motions, orbital eccentricitiesare computed for 72 stars not already considered in a similar study ofsouthern stars by Norris et al. (1985). A few stars with e of less than0.4 are found.
| Objective-prism discoveries in the northern sky - II.
Five tables list the observed characteristics of 244 northern hemispherestars recorded in 100 108 A/mm dispersion plates at H-gamma wavelengths.The 5 x 5 deg fields contained a number of suspected weak-metal objects.Listings are divided into peculiar A stars, metallic line and deltaDelphini stars, high luminosity objects, Ca II emission stars of the Gand K type, and composite spectra.
| Luminous stars in a region south of h and chi Persei. With 3 Figures in the Text
Not Available
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Андромеда |
| Прямое восхождение: | 02h26m49.74s |
| Склонение: | +44°57'46.5" |
| Видимая звёздная величина: | 9.114 |
| Собственное движение RA: | 121.1 |
| Собственное движение Dec: | -34.1 |
| B-T magnitude: | 9.745 |
| V-T magnitude: | 9.167 |
Каталоги и обозначения:
|
