目录
图像
上传图像
DSS Images Other Images
相关文章
Spectral Energy Distributions of M81 Globular Clusters in the BATC Multicolor Survey
In this paper, we give the spectral energy distributions (SEDs) of 42M81 globular clusters in 13 intermediate-band filters from 4000 to 10000Å using the CCD images of M81, observed as part of theBeijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey. TheBATC multicolor filter system is specifically designed to exclude mostof the bright and variable night-sky emission lines, including the OHforest. Hence, it can present accurate SEDs of the observed objects.These SEDs are low-resolution spectra and can reflect the stellarpopulations of the globular clusters. This paper confirms theconclusions of Schroder et al., that M81 contains clusters as young as afew Gyr, which were also observed in both M31 and M33.
| Optical multiband observations of BL Lacertae during the outburst of 2005
The aim of our observations is to investigate the intranight variabilityproperties and the spectral variability of BL Lacertae. 799 opticalmultiband observations were intensively made with theBeijing-Arizona-Taiwan-Connecticut (BATC) 60/90 cm Schmidt telescopeduring the outburst composed of two subsequent flares in 2005. Thesecond flare, whose rising phase lasted at least 44 d, was observed withamplitudes of more than 1.1 mag in three BATC optical bands. In general,on intranight time-scale the amplitude of variability and the variationrate are larger at the shorter wavelength, and the variation rate iscomparable in the rising and decaying phases within each band. Apossible time-lag between the light curves in bands e and m, around 11.6min, was obtained. Based on the analysis of the colour index variationwith the source brightness, the variability of BL Lacertae can beinterpreted as having two components: a `strongly chromatic' intranightcomponent and a `mildly chromatic' internight component, which may bethe results of both intrinsic physical mechanism and geometric effects.
| Galactic structure studies from the Beijing-Arizona-Taiwan-Connecticut survey
We present an analysis of the photometric parallaxes of stars in 21 ofthe Beijing-Arizona-Taiwan-Connecticut Survey fields carried out withthe National Astronomical Observatories (NAOC) 60/90 cm SchmidtTelescope in 15 intermediate-band filters from 3000 to 10000 Å. Inthis study, we have adopted a three-component (thin disc, thick disc andhalo) model to analyse star count information. By calculating thestellar space density as a function of distance from the Galactic plane,we determine that the range of scaleheight for the thin disc varies from220 to 320 pc. Although 220 pc seems an extreme value, it is close tothe lower limit in the literature. The range of scaleheight for thethick disc is from 600 to 1100 pc, and the corresponding space numberdensity normalization is 7.0-1.0 per cent of the thin disc. We find thatthe scaleheight of the disc may be variable with the observed direction,which cannot simply be attributed to statistical errors. Possibly themain reasons can be attributed to the disc (mainly the thick disc) beingflared, with a scaleheight increasing with radius. The structure isconsistent with the merger origin for the thick disc formation. Adoptinga de Vaucouleurs r1/4 law halo, we also find that the axisratio towards the Galactic Centre is somewhat flatter (~0.4), while theshape of the halo in the anticentre and antirotation direction isrounder with c/a > 0.4. Our results show that star counts indifferent lines of sight can be used directly to obtain a rough estimateof the shape of the stellar halo. Our solutions support the Galacticmodels with a flattened inner halo, possibly formed by an early mergerin the Galaxy's history.
| New age estimates of M31 globular clusters from multicolour photometry
The large majority of extragalactic star cluster studies performed todate essentially use multicolour photometry, combined with theoreticalstellar synthesis models, to derive ages, masses, extinction estimatesand metallicities. M31 offers a unique laboratory for studies ofglobular cluster (GC) systems. In this paper, we obtain new ageestimates for 91 M31 GCs, based on improved photometric data, updatedtheoretical stellar synthesis models and sophisticated new fittingmethods. In particular, we used photometric measurements from the TwoMicron All Sky Survey (2MASS), which, in combination with opticalphotometry, can partially break the well-known age-metallicitydegeneracy operating at ages in excess of a few Gyr. We show robustlythat previous age determinations based on photometric data were affectedsignificantly by this age-metallicity degeneracy. Except for onecluster, the ages of our other sample GCs are all older than 1 Gyr.Their age distribution shows populations of young- and intermediate-ageGCs, peaking at ~3 and 8 Gyr, respectively, as well as the `usual'complement of well-known old GCs, i.e. GCs of similar age as themajority of the Galactic GCs. Our results also show that although thereis significant scatter in metallicity at any age, there is a notablelack of young metal-poor and old metal-rich GCs, which might beindicative of an underlying age-metallicity relationship among the M31GC population.
| Optical spectroscopy of the dwarf nova U Geminorum
The dwarf nova U Gem is unique in having a direct measurement of the Kvelocity of its white dwarf from Hubble Space Telescope (HST) spectra(K1 = 107 +/- 2kms-1 from Long et al.). We presenthigh-resolution optical spectra of U Gem in quiescence taken to test theaccuracy to which the HST value can be recovered from optical data. Wefind that, even with data of very high signal-to-noise ratio on thisarchetypal system, we cannot recover Long et al.'s value to better thanabout 20 per cent by any method. Contamination by neighbouring emissionlines seems a likely culprit. Our data reveal a number of new features:Doppler tomograms show emission at low velocity, close to the centre ofmass, and a transient, narrow absorption feature is seen in the Balmerlines near the line centres at the time of eclipse. We suggest thatstellar prominences, as previously invoked for the dwarf novae IP Pegand SS Cyg in outburst, may explain both of these features. The HeII4686.75 Å line emission is dominated by the gas stream/disc impactregion. Two distinct spots are seen in Doppler maps, the first beingvery narrow and showing a velocity close to that of the accretion discin the impact region, and the second much broader and located betweenthe velocities of the (ballistic) stream and the (Keplerian) disc. Wepresent tentative evidence of weak spiral structure, which may supportexplanations for `spiral shocks' based upon three-body effects. We findno evidence of stream-disc overflow in the system. Our data suggest aninclination angle >70°, favouring a mass <1.1Msolarfor the white dwarf, in disagreement with measurements of itsgravitational redshift. The mass donor is clearly seen in the Dopplermaps, with emission concentrated towards its poles, and mainly on theside facing the white dwarf. This suggests irradiation with shielding bythe disc from which we estimate a height-to-radius ratio (H/R) between0.15 and 0.25.
| Simultaneous photometry and echelle spectroscopy of the dwarf nova BZ Ursae Majoris during the 2005 January outburst
We report simultaneous photometric and echelle-spectroscopicobservations of the dwarf nova BZ UMa during which we were lucky to catchthe system at the onset of an outburst, the development of which wetraced in detail from quiescence to early decline. The outburst had aprecursor, and was of a short duration (~5d) with a highly asymmetricallight curve. On the rise we observed a `jump' during which thebrightness almost doubled over the course of half an hour. Power spectraanalysis revealed well-defined oscillations with period of ~42min. UsingDoppler tomography we found that the unusual emission distributiondetected in quiescence held during the outburst. After the maximum a newemission source arose, from the inner hemisphere of the secondary star,which became the brightest at that time. We analyse this outburst interms of `inside-out' and `outside-in' types, in order to determinewhich of these types occurred in BZ UMa.
| A `super' star cluster grown old: the most massive star cluster in the Local Group
We independently redetermine the reddening and age of the globularcluster (GC) 037-B327 in M31 by comparing independently obtainedmulticolour photometry with theoretical stellar population synthesismodels. 037-B327 has long been known to have a very large reddeningvalue, which we confirm to be E(B - V) = 1.360 +/- 0.013, in goodagreement with the previous results. We redetermine its most likely ageat 12.4 +/- 3.2 Gyr.037-B327 is a prime example of an unusually bright early counterpart tothe ubiquitous `super' star clusters presently observed in mosthigh-intensity star-forming regions in the local Universe. In order tohave survived for a Hubble time, we conclude that its stellar initialmass function (IMF) cannot have been top-heavy. Using this constraint,and a variety of simple stellar population (SSP) models, we determine aphotometric mass of , somewhat depending on the SSP models used, themetallicity and age adopted and the IMF representation. This mass, andits relatively small uncertainties, makes this object the most massivestar cluster of any age in the Local Group. Assuming that thephotometric mass estimate thus derived is fairly close to its dynamicalmass, we predict that this GC has a (one-dimensional) velocitydispersion of the order of (72 +/- 13) km s-1. As a surviving`super' star cluster, this object is of prime importance for theoriesaimed at describing massive star cluster evolution.
| Optical and infrared observations of the TypeIIP SN2002hh from days 3 to 397
We present optical and infrared (IR) observations of the TypeII SN2002hhfrom 3 to 397d after explosion. The optical spectroscopic (4-397d) andphotometric (3-278d) data are complemented by spectroscopic (137-381d)and photometric (137-314d) data acquired at IR wavelengths. This is thefirst time L-band spectra have ever been successfully obtained for asupernova (SN) at a distance beyond the Local Group. The VRI lightcurves in the first 40d reveal SN2002hh to be an SNIIP (plateau) - themost common of all core-collapse SNe. SN2002hh is one of the most highlyextinguished SNe ever investigated. To provide a match between itsearly-time spectrum and a coeval spectrum of the TypeIIP SN1999em, aswell as maintaining consistency with KI interstellar absorption, weinvoke a two-component extinction model. One component is due to thecombined effect of the interstellar medium (ISM) of our Milky Way Galaxyand the SN host galaxy, while the other component is due to a `dustpocket' where the grains have a mean size smaller than in the ISM. Theearly-time optical light curves of SNe1999em and 2002hh are generallywell matched, as are the radioactive tails of these two SNe and SN1987A.The late-time similarity of the SN2002hh optical light curves to thoseof SN1987A, together with measurements of the optical/IR luminosity and[FeII]1.257μm emission indicate that 0.07 +/- 0.02Msolarof 56Ni was ejected by SN2002hh. However, during the nebularphase the HKL' luminosities of SN2002hh exhibit a growing excess withrespect to those of SN1987A. We attribute much of this excess to anIR-echo from a pre-existing, dusty circumstellar medium. Based on anIR-echo interpretation of the near-IR (NIR) excess, we deduce that theprogenitor of SN2002hh underwent recent mass-loss of~0.3Msolar. A detailed comparison of the late-time opticaland NIR spectra of SNe1987A and 2002hh is presented. While the overallimpression is one of similarity between the spectra of the two events,there are notable differences. The MgI1.503μm luminosity of SN2002hhis a factor of 2.5 greater than in SN1987A at similar epochs, yet coevalsilicon and calcium lines in SN2002hh are fainter. Interpreting thesedifferences as being due to abundance variations, the overall abundancetrend between SN1987A and 2002hh is not consistent with explosion modelpredictions. It appears that during the burning to intermediate-masselements, the nucleosynthesis did not progress as far as might have beenexpected given the mass of iron ejected. Evidence for mixing in theejecta is presented. Pronounced blueshifts seen in the more isolatedlines are attributed to asymmetry in the ejecta. However, during thetime-span of these observations (~1-yr post-explosion) we find noevidence of dust condensation in the ejecta such as might have beenrevealed by an increasing blueshift and/or attenuation of the red wingsof the emission lines. Nevertheless, the clear detection of firstovertone CO emission by 200d and the reddening trend in (K -L')0 suggest that dust formation in the ejecta may occur atlater epochs. From the [OI] λλ6300, 6364Å doubletluminosity we infer a 16-18Msolar main-sequence progenitorstar. The progenitor of SN2002hh was probably a red supergiant with asubstantial, dusty wind.
| Lithium Isotopic Abundances in Metal-poor Halo Stars
Very high quality spectra of 24 metal-poor halo dwarfs and subgiantshave been acquired with ESO's VLT/UVES for the purpose of determining Liisotopic abundances. The derived one-dimensional, non-LTE 7Li abundances from the Li I 670.8 nm line reveal a pronounceddependence on metallicity but with negligible scatter around this trend.Very good agreement is found between the abundances from the Li I 670.8nm line and the Li I 610.4 nm line. The estimated primordial 7Li abundance is7Li/H=(1.1-1.5)×10-10, which is a factor of3-4 lower than predicted from standard big bang nucleosynthesis with thebaryon density inferred from the cosmic microwave background.Interestingly, 6Li is detected in 9 of our 24 stars at the>=2 σ significance level. Our observations suggest theexistence of a 6Li plateau at the level oflogε6Li~0.8 however, taking into accountpredictions for 6Li destruction during the pre-main-sequenceevolution tilts the plateau such that the 6Li abundancesapparently increase with metallicity. Our most noteworthy result is thedetection of 6Li in the very metal-poor star LP 815-43. Sucha high 6Li abundance during these early Galactic epochs isvery difficult to achieve by Galactic cosmic-ray spallation andα-fusion reactions. It is concluded that both Li isotopes have apre-Galactic origin. Possible 6Li production channels includeprotogalactic shocks and late-decaying or annihilating supersymmetricparticles during the era of big bang nucleosynthesis. The presence of6Li limits the possible degree of stellar 7Lidepletion and thus sharpens the discrepancy with standard big bangnucleosynthesis.Based on observations collected at the European Southern Observatory,Paranal, Chile (observing programs 65.L-0131, 68.D-0091, and273.D-5043).
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Multicolor Photometry and Stellar Population Synthesis Study of the Interacting Galaxies of the Leo Triplet
This paper studies the nearby interacting group the Leo Triplet usingmulticolor photometry data obtained with the National AstronomicalObservatories of China 60 cm/90 cm Schmidt telescope. The observationcovers the entire area of the three galaxies (NGC 3623, NGC 3627, andNGC 3628), and spectral energy distributions (SEDs) of each galaxy areobtained. By comparing the observed SEDs of each part of the galaxieswith the theoretical ones generated by instantaneous burst evolutionarysynthesis models with different metallicities (Z=0.0001, 0.008, 0.02,and 0.05), two-dimensional relative age distribution maps of the threegalaxies were obtained. NGC 3623 exhibits a very weak age gradient fromthe bulge to the disk. This gradient is absent in NGC 3627. The ages ofthe dominant stellar populations of NGC 3627 and NGC 3628 areconsistent, and this consistency is model independent (0.5-0.6 Gyr,Z=0.02), but the ages of NGC 3623 are systematically older (0.7-0.9 Gyr,Z=0.02). The results indicate that NGC 3627 and NGC 3628 have undergonesynchronous evolution and that the interaction has likely triggeredstarbursts in both galaxies. For NGC 3623, however, the weak agegradient may indicate recent star formation in its bulge, which hascaused its color to turn blue. Evidence is found for a potential barexisting in the bulge of NGC 3623, and my results support the view thatNGC 3623 does interact with NGC 3627 and NGC 3628.
| Optical Monitoring of BL Lacertae Object OJ 287: A 40 Day Period?
We present the results of our optical monitoring of the BL Lacertaeobject OJ 287 during the first half of 2005. The source did not showlarge-amplitude variations during this period and was in a relativelyquiescent state. A possible period of 40 days was derived from its lightcurves in three BATC wave bands. A bluer-when-brighter chromatism wasdiscovered, which is different from the extremely stable color duringthe outburst in 1994-1996. The different color behaviors imply differentvariation mechanisms in the two states. We then revisited the opticaldata on OJ 287 from the OJ-94 project and also found a probable periodof 40 days in its optical variability during the late 1994 outburst. Theresults suggest that two components contribute to the variability of OJ287 during its outburst state. The first component is the normal blazarvariation. This component has an amplitude similar to that of thequiescent state and also may share a similar periodicity. The secondcomponent can be taken as a ``low-frequency modulation'' to the firstcomponent. It may be induced by the interaction of the assumed binaryblack holes in the center of this object. The 40 day period may berelated to the helical structure of the magnetic field at the base ofthe jet or to the orbital motion close to the central primary blackhole.
| Optical Photometry of the Type II-P Supernova 2004dj in NGC 2403
We present photometric data of the Type II-P supernova (SN) 2004dj inNGC 2403. The multicolor light curves cover the SN from ~60 to 200 daysafter explosion and are measured with a set of intermediate-band filtersthat have the advantage of tracing the strength variations of somespectral features. The light curves show a flat evolution in the middleof the plateau phase, then decline exponentially at the late times, witha rate of 0.10+/-0.03 mag (10 days)-1 in most of the filters.In the nebular phase, the spectral energy distribution of SN 2004djshows a steady increase in the flux near 6600 and 8500 Å, whichmay correspond to the emission lines of Hα and the Ca II near-IRtriplet, respectively. The photometric behavior suggests that SN 2004djis a normal SN II-P. Compared with the light curves of another typicalSN II-P, 1999em, we estimate the explosion date to be 2004 June 10+/-21UT (JD 2,453,167+/-21) for SN 2004dj. We also estimate the ejectednickel mass during the explosion to be M(56Ni)=0.023+/-0.005Msolar from two different methods, which is typical for a SNII-P. We derive the explosion energy to beE~0.75+0.56-0.38×1051 ergs, theejecta mass to be M~10.0+7.4-5.2Msolar, and the initial radius to beR~282+253-122 Rsolar for thepresupernova star of SN 2004dj, which are consistent with other typicalSNe II-P.
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Timescales of merger, starburst and AGN activity in radio galaxy B2 0648+27
We use neutral hydrogen (H I) and optical spectroscopic observations tocompare the timescales of a merger event, starburst episode andradio-AGN activity in the radio galaxy B2 0648+27. Wedetect a large ring-like structure of H I in emission around theearly-type host galaxy of B2 0648+27 (MH I= 8.5 × 109~Mȯ, diameter = 190 kpc). Weinterpret this as the result of a major merger that occurred ≳1.5Gyr ago. From modelling optical long-slit spectra we find that a youngstellar population of 0.3 Gyr, indicative of a past starburst event,dominates the stellar light throughout the galaxy. The off-set in timebetween the merger event and the starburst activity in B20648+27 suggests that the starburst was triggered in anadvanced stage of the merger, which can be explained if the gas-richprogenitor galaxies contained a bulge. Although the exact age of theradio source remains uncertain, there appears to be a significanttime-delay between the merger/starburst event and the current episode ofradio-AGN activity. We also observe an outflow of emission-line gas inthis system, which is likely related to superwinds driven by the starsthat formed during the starburst event. We argue that the radio galaxyB2 0648+27 is a link in the evolutionary sequencebetween Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-typegalaxies.
| Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
Aims.Parameters for 55 nearby metal-poor stars are determined usinghigh-resolution spectroscopy. Together with similar data taken from arecent analysis, they are used to show trends of their Galacticevolution with stellar [Fe/H] or [Mg/H] abundances. The separation ofabundance ratios between disk and halo stars is used as a basiccriterion for population membership. Methods.After carefulselection of a clean subsample free of suspected or known binaries andpeculiar stars, abundances of Mg, Na and Al are based on NLTE kineticequilibrium calculations applied to spectrum synthesis methods. Results.The relation between [Na/Mg] and [Fe/H] is a continuousenrichment through all three Galactic populations spanning a range ofvalues between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values.[Al/Mg] displays a step-like difference between stars of the Galactichalo with overline[Al/Mg] -0.45 and the two disk populations withoverline[Al/Mg] +0.10. [Al/Mg] ratios, together with the [Mg/Fe]ratios, asymmetric drift velocities V, and stellar evolutionary ages,make possible the individual discrimination between stars of the thickdisk and the halo. At present, this evidence is limited by the smallnumber of stars, and by the theoretical and empirical uncertainties ofstellar age determinations, but it achieves a high significance. Conclusions.While the stellar sample is not complete with respect tospace volume, the resulting abundances indicate the necessity to revisecurrent models of chemical evolution to allow for an adequate productionof Al in early stellar generations.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Age estimations of M 31 globular clusters from their spectral energy distributions
This paper presents accurate spectral energy distributions (SEDs) of 15M 31 globular clusters (GCs) confirmed by spectroscopy and/or highspatial-resolution imaging, as well as 30 M 31 globular clustercandidates detected by Mochejska et al. (1998, AcA, 48, 455). Most ofthese candidates have mV > 18, deeper than previoussearches, and these candidates have not yet been confirmed to beglobular clusters. The SEDs of these clusters and candidates areobtained as part of the BATC Multicolor Survey of the Sky, in which thespectrophotometrically-calibrated CCD images of M 31 in 13intermediate-band filters from 4000 to 10 000 Å were observed.These filters are specifically designed to exclude most of the brightand variable night-sky emission lines including the OH forest. Incomparison to the SEDs of true GCs, we find that some of the candidateobjects are not GCs in M 31. SED fits show that theoretical simplestellar population (SSP) models can fit the true GCs very well. Weestimate the ages of these GCs by comparing with SSP models. We findthat, the M 31 clusters range in age from a few ten Myr to a few Gyrold, as well as old GCs, confirming the conclusion that has been foundby Barmby et al. (2000, AJ, 119, 727), Williams & Hodge (2001, ApJ,548, 190), Beasley et al. (2004, AJ, 128, 1623), Burstein et al. (2004,ApJ, 614, 158) and Puzia et al. (2005, A&A, 434, 909) in theirinvestigations of the SEDs of M 31 globular clusters.
| The measurement of instrument efficiency for the BATC photometric system
Four Oke-Gunn (OG) standards, HD 19445, HD 84937, BD +17°4708 and BD+26°2606 are used as standard stars for flux calibration in the BATCproject. We use the BATC 14-color filters among the BATC 15intermediate-band filters (except b band) for photometry of the skyregions of the two standard stars: HD 19445 and HD 84937. We also usetwo automatic data reduction programs Pipeline I and Pipeline II toprocess observational data and calibrate the BATC magnitudes of theother stars in the observed sky regions with the known BATC standardstar. In this paper, we have evaluated the sky brightness, found therelationship among the signal-to-noise ratio, the BATC magnitudes andthe exposure time, and got the experimental formula among the threequantities.
| Identification of X-Ray Point Sources and a Study of the Nature of 62 X-Ray Globular Cluster Candidates in M31
This paper includes two parts. The first presents the spectral energydistributions (SEDs) of 49 globular cluster (GC) X-ray sources in 13BATC (Beijing-Arizona-Taiwan-Connecticut) intermediate-band filters from3800 to 10000 Å, and identifies 8 previously unidentified X-raysources in M31. Using X-ray data from Einstein observations from 1979 to1980, ROSAT High Resolution Imager observations in 1990, Chandra HighResolution Channel and ACIS-I observations from 1999 to 2001, and theBATC optical survey from 1995 to 1999, we find 49 GC X-ray sources and 8new X-ray sources in the BATC M31 field. By analyzing SEDs and FWHMs, wedetermine that 4 of the 8 X-ray sources may be GC candidates. The secondpart presents some statistical relationships between 62 GC X-raysources, of which 58 are already known and 4 are identified in thispaper. The distribution of M31's GC X-ray sources' V magnitudes isbimodal, with peaks at mv=15.65 and 17.89, which is differentfrom the distribution of GC candidates. The distribution of B-V colorshows that the GC X-ray sources seem to be associated preferentiallywith the redder GCs, in agreement with previous results.Kolmogorov-Smirnov test shows that the maximum value of the absolutedifference of B-V distributions of GC X-ray sources and GCs isDmax=0.181, and the probability P=0.068, which means we canreject the hypothesis that the two distributions are the same at the90.0% confidence level. Finally, we study the correlation between X-rayluminosity (0.3-10 keV) and optical luminosity (in the V band) of the GCX-ray sources in M31, and find that there exits a weak relationship withthe linear correlation coefficient r=0.36 at a confidence level of98.0%.
| Thirteen-Color Photometry of Open Cluster M48
This paper presents 13 color CCD intermediate-band spectrophotometry ofa field centered on the open cluster M48 (NGC 2548) from 400 to nearly1000 nm, taken with Beijing-Arizona-Taiwan-Connecticut (BATC) ColorSurvey photometric system. The fundamental parameters of this clusterare derived with a new method that is based on a comparison of thespectral energy distributions (SEDs) of cluster stars and thetheoretical SEDs of Padova models. We find that the best-fitting age ofM48 is 0.32 Gyr, with a distance of 780 pc, a reddening of E(B-V)=0.04,and a solar metallicity of Z=0.019.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| Lithium isotopic abundances in metal-poor stars: A problem for standard big bang nucleosynthesis?
Not Available
| Chemical abundances of very metal-poor stars
High-resolution and high signal-to-noise ratio spectra of 32 verymetal-poor stars were obtained with the Coudé echellespectrograph mounted on the 2.16-m telescope at the NationalAstronomical Observatories (Xinglong, China). Equivalent widths of FeI,FeII, OI, NaI, MgI, AlI, SiI, SiII, KI, CaI, ScII, TiI, VI, CrI, MnI,NiI, CuI and BaII lines were measured. Stellar effective temperatureswere determined by colour indices. Stellar surface gravities werecalculated from Hipparcos parallaxes and stellar evolutionary tracks.Photospheric abundances of 16 elements were derived by localthermodynamical equilibrium analysis. Stellar space motions (U, V, W)and Galactic orbital parameters were calculated. Based on kinematics,sample stars were separated into dissipative collapse and accretioncomponents of halo population. The global kinematics of the twocomponents were analysed. Element abundances were discussed as functionsof metallicities. The results of oxygen and α-elements abundanceconfirmed the previous works. The [K/Fe] shows a gradual systematicincrease toward a lower metallicity, such as in the case ofα-elements. The [Ba/Fe] trend suggests that the s-processdominated Ba production at least for the metal-poor stars with[Fe/H]> -2.0.
| Spectral evolution of V838 Monocerotis in the optical and near-infrared in early 2002
We report optical and near-infrared spectroscopy, and opticalspectropolarimetry, of the peculiar variable V838 Mon during themultiple outburst phase in early 2002. The spectral evolution isexceptional. Our earliest spectra (2002 January) are noteworthy fortheir strong absorption lines of barium and strontium in the optical,and bands of CO and circumstellar H2O in the near-infrared.All but the CO weaken or are absent in later spectra. The behaviour ofthe CO band during this phase is extraordinary: initially in absorption,it was observed two months later in optically thick emission. Theexcitation of the CO is probably the result of the propagation of ashock wave at the third maximum. The two spectropolarimetric epochs weretaken 6 and 27 d after the second outburst on 2002 February 8. Thepolarization at both times was measured to be pV~ 2.7 percent. Nearly all of the measured polarization is believed to be due tointerstellar dust, a conclusion that is consistent with previousstudies. At both epochs, however, a weak and variable intrinsiccomponent is thought to be present. Between January and March of 2002the luminosity of V838 Mon increased by a factor of 15 and the apparentdiameter increased fourfold.
| Spectroscopy of the γ-ray burst GRB 021004: a structured jet ploughing through a massive stellar wind
We present spectra of the afterglow of the γ-ray burst GRB 021004taken with the ISIS spectrograph on the William Herschel Telescope (WHT)and with the Focal Reducer/Low Dispersion Spectrograph 1 (FORS1) on theVery Large Telescope (VLT) at three epochs spanning 0.49-6.62 d afterthe burst. We observe strong absorption probably coming from the hostgalaxy, alongside absorption in HI, SiIV and CIV with blueshifts of upto 2900 km s-1 from the explosion centre, which we assumeoriginates close to the progenitor. We find no significant variabilityof these spectral features. We investigate the origin of the outflowingmaterial and evaluate various possible progenitor models. The mostplausible explanation is that these result in the fossil stellar wind ofa highly evolved Wolf-Rayet (WR) star. However, ionization from theburst itself prevents the existence of HI, SiIV and CIV close to theafterglow surface where the fast stellar wind should dominate, and largeamounts of blueshifted hydrogen are not expected in a WR star wind. Wepropose that the WR star wind is enriched by a hydrogen-rich companion,and that the GRB has a structured jet geometry in which the γ-raysemerge in a small opening angle within the wider opening angle of thecone of the afterglow. This scenario is able to explain both thespectral-line features and the irregular light curve of this afterglow.
| The Discovery and Analysis of Very Metal-Poor Stars in the Galaxy
We discuss the importance of very metal-poor stars to develop anunderstanding of the nature of the first stars that formed in theUniverse and the nucleosynthesis events associated with them, as well asto refine models of galaxy formation, in particular for large spiralgalaxies such as the Milky Way. After briefly reviewing the history ofthe search for very metal-deficient stars in the Galaxy, we summarizeongoing efforts, concentrating on the two large objective-prism surveysthat have led to the discovery of the majority of stars with [Fe/H] < 2.0 known at present: the HK survey of Beers and collaborators and theHamburg/ESO survey of Christlieb and collaborators. We then consider thewealth of information that can be gleaned from high-resolutionspectroscopic study of very metal-poor stars. We close with a list ofopen questions and a discussion of new survey techniques that willexpand the sample of recognized very metal-deficient stars in the Galaxyby several orders of magnitude.
| New Light on Stellar Abundance Analyses: Departures from LTE and Homogeneity
The information on the chemical compositions of stars encoded in theirspectra plays a central role in contemporary astrophysics. Stellarelement abundances are, however, not observed: to decipher the spectralfingerprints in terms of abundances requires realistic models for thestellar atmospheres and the line-formation processes. Still today, thevast majority of abundance analyses of late-type stars rely onone-dimensional (1D), hydrostatic model atmospheres and the assumptionof local thermodynamic equilibrium (LTE). In this review possiblesystematic errors in studies of F-, G- and K-type stars introduced bythese questionable approximations are discussed. Departures from LTE arecommonplace and often quite severe, in particular for low surfacegravities or metallicities, with minority species and low-excitationtransitions being the most vulnerable. Recently, time-dependent, 3D,hydrodynamical model atmospheres have started to be employed for stellarabundance purposes, with large differences compared with 1D modelingfound in particular for metal-poor stars. An assessment of non-LTE and3D effects for individual elements as well as on the estimated stellarparameters is presented.
| The Progenitor of SN 2004dj in a Star Cluster
The progenitor of Type II-P (P = plateau) supernova SN 2004dj isidentified with a supergiant in a compact star cluster known as``Sandage's star 96'' (S96) in the nearby spiral galaxy NGC 2403, whichwas fortuitously imaged as part of theBeijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey from1995 February to 2003 December prior to SN 2004dj. The superiorphotometry of BATC images for S96, taken with 14 intermediate-bandfilters covering 3000-10000 Å, unambiguously establishes the starcluster nature of S96, with an age of ~20 Myr, a reddening ofE(B-V)~0.35 mag, and a total mass of ~96,000 Msolar. Thecompact star cluster nature of S96 is also consistent with the lack oflight variations in the past decade. The SN progenitor is estimated tohave a main-sequence mass of ~12 Msolar. The comparison ofour intermediate-band data of S96 with the postoutburst photometryobtained as the SN significantly dims with time may hopefullyconclusively establish the nature of the progenitor.
| A Definitive Measurement of Time Dilation in the Spectral Evolution of the Moderate-Redshift Type Ia Supernova 1997ex
We have obtained high-quality Keck optical spectra at three epochs ofthe Type Ia supernova 1997ex, whose redshift z is 0.361. The elapsedcalendar time between the first two spectra was 24.88 days, and thatbetween the first and third spectra was 30.95 days. In an expandinguniverse where 1+z represents the factor by which space has expandedbetween the emission and detection of light, the amount of aging in thesupernova rest frame should be a factor of 1/(1+z) smaller than theobserved-frame aging; thus, we expect SN 1997ex to have aged 18.28 and22.74 days between the first epoch and the second and third epochs,respectively. The quantitative method for determining thespectral-feature age of an SN Ia reveals that the corresponding elapsedtimes in the supernova rest frame were 16.97+/-2.75 and 18.01+/-3.14days, respectively. This result is inconsistent with no time dilationwith a significance level of 99.0%, providing evidence against ``tiredlight'' and other hypotheses in which no time dilation is expected.Moreover, the observed timescale of spectral evolution is inconsistentwith that expected in the ``variable mass theory.'' The result is within~1 σ of the aging expected from a universe in which redshift isproduced by cosmic expansion.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
