目录
图像
上传图像
DSS Images Other Images
相关文章
Comparative statistics and origin of triple and quadruple stars
The statistics of catalogued quadruple stars consisting of two binaries(hierarchy 2 + 2), is studied in comparison with triple stars, withrespective sample sizes of 81 and 724. Seven representative quadruplesystems are discussed in greater detail. The main conclusions are asfollows. (i) Quadruple systems of ? Lyr type with similar massesand inner periods are common, in 42 per cent of the sample the outermass ratio is above 0.5 and the inner periods differ by less than 10times. (ii) The distributions of the inner periods in triple andquadruple stars are similar and bimodal. The inner mass ratios do notcorrelate with the inner periods. (iii) The statistics of outer periodsand mass ratios in triples and quadruples are different. The medianouter mass ratio in triples is 0.39 independently of the outer period,which has a smooth distribution. In contrast, the outer periods of 25per cent quadruples concentrate in the narrow range from 10 to 100yr,the outer mass ratios of these tight quadruples are above 0.6 and theirtwo inner periods are similar to each other. (iv) The outer and innermass ratios in triple and quadruple stars are not mutually correlated.In 13 per cent of quadruples both inner mass ratios are above 0.85(double twins). (v) The inner and outer orbital angular momenta andperiods in triple and quadruple systems with inner periods above 30dshow some correlation, the ratio of outer-to-inner periods is mostlycomprised between 5 and 104. In the systems with small periodratios the directions of the orbital spins are correlated, while in thesystems with large ratios they are not. The properties of multiple starsdo not correspond to the products of dynamical decay of small clusters,hence the N-body dynamics is not the dominant process of theirformation. On the other hand, rotationally driven (cascade)fragmentation possibly followed by migration of inner and/or outerorbits to shorter periods is a promising scenario to explain the originof triple and quadruple stars.
| Pre-main sequence spectroscopic binaries suitable for VLTI observations
Context: A severe problem for research in star-formation is that themasses of young stars are almost always estimated from evolutionarytracks alone. Since the tracks published by different groups differ, itis often only possible to give a rough estimate of the masses of youngstars. It is thus crucial to test and calibrate the tracks. Up to now,only a few tests of the tracks could be carried out. However, it is nowpossible with the VLTI to set constrains on the tracks by determiningthe masses of many young binary stars precisely. Aims: In order to usethe VLTI efficiently, a first step is to find suitable targets, which isthe purpose of this work. Given the distance of nearby star-formingregions, suitable VLTI targets are binaries with orbital periods betweenat least 50 days and a few years. Although a number of surveys fordetecting spectroscopic binaries have been carried out, most of thebinaries found so far have periods that are too short. Methods: We thussurveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρOphiuci star-forming regions in order to search for spectroscopicbinaries with periods longer than 50 days, which are suitable for theVLTI observations. Results: As a result of the 8 year campaign, wediscovered 8 binaries with orbital periods longer than 50 days. Amongstthe newly discovered long-period binaries is CS Cha, which is one of thefew classical T Tauri stars with a circumbinary disk. The survey islimited to objects with masses higher than 0.1 to 0.2 M_ȯ forperiods between 1 and 8 years. Conclusions: We find that the frequencyof binaries with orbital periods ≤3000 days is of 20±5%. Thefrequency of long and short period pre-main sequence spectroscopicbinaries is about the same as for stars in the solar neighbourhood. Intotal 14 young binaries are now known that are suitable for massdetermination with the VLTI.based on observations obtained at the European Southern Observatory atLa Silla, Chile in program 62.I-0418(A); 63.I-0096(A); 64.I-0294(A);65.I-0012(A); 67.C-0155(A); 68.C-0292(A); 68.C-0561(A); 69.C-0207(A);70.C-0163(A); 073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F). Tables 2,3, 5, 6, 8, 10, 12-20 are only available in electronic form athttp://www.aanda.org
| 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.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association
We report spectroscopic observations of stars belonging to the youngnearby group known as the TW Hydrae association, as well as of a numberof potential members of the association identified in kinematic andX-ray surveys. Multiple radial velocity measurements were obtained foreach object, several of which turn out to be multiple systems. Orbitalsolutions are presented for three double-lined binaries, onesingle-lined binary, and a double-lined triple system, all with shortperiods. Effective temperatures and projected rotational velocities arepresented for each visible object. None of the candidate members of theassociation in our sample are confirmed as a true member. The largefraction of close binaries among the candidate members has to do withtheir selection based on X-ray emission from ROSAT, which tends to favorthe inclusion of tidally locked systems that are active but notnecessarily young.Some of the observations reported here were obtained with the MultipleMirror Telescope, a joint facility of the Smithsonian Institution andthe University of Arizona.
| F, G and K stars in the ROSAT all-sky survey. I. Photometry
We present accurate BV(RI)_c photometry for a sample of F, G and K starsdetected in selected areas of the ROSAT all-sky survey (RASS). We haveused the photometry, in addition to low-resolution spectroscopy, toestimate spectral classifications, distances and X-ray luminosities. Thelog (L_X / L_V) in the sample lies below -2. Although the samplecontains also nearby, inactive stars, it is dominated by active objects.The median X-ray luminosity in our sample is < L_X > = 29.88 andthe mean value of the hardness ratios = 0.13 +/- 0.35. Wecompare the derived X-ray luminosity function with similar functionsobtained from the serendipitous samples of the Einstein Observatorymedium sensitivity survey (EMSS) and EXOSAT. Our sample is completelyconsistent with the EMSS sample of solar type stars, indicating thatboth our sources and the EMSS sources are representative of the highgalactic latitude X-ray stellar population. We do not find extremelyactive stars (log (L_X) >= 32), as are found in the EMSS sample, andwe argue that these objects are rare. Table 3 is also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| The nature of isolated T Tauri stars
We present the results of a search for young stellar objects around thetwo isolated T Tauri stars (TTSs) TW Hya and CoD-29degr 8887. From thespectroscopic properties of these two objects, it is obvious that theyare T Tauri stars, although they are not associated with a star-formingregion as it is the case for most of the known TTSs. Especially TW Hyais the only classical TTS that is not located in a dark cloud with starformation activity. The same is true for the weak-line T Tauri starCoD-29degr 8887. We searched for pre-main sequence stars using ROSATPSPC observations pointing at our two main targets. With a sophisticatedsearch strategy we could identify 107 X-ray sources in our fields. Forthe 37 stellar-like optical counterparts we did spectroscopic follow-upobservations. These show that within the viewing field of ROSAT thereare no other X-ray emitting young stellar objects around TW Hya andCoD-29degr 8887. For the isolated TTSs TW Hya and HD 98800 Hipparcosparallax measurements are available locating them at distances of 56 and46pc, respectively. This means that they are the closest TTSs withcircumstellar dusty disks known today. The space velocities of these twoobjects are 3-5km/sec. From their position in the HR diagram, weobtained an age of about 10(7) years. Hence, we conclude that these twoobjects travelled not far away from their original birthplace and theirparental molecular cloud dispersed meanwhile. This is the solution whythese T Tauri stars appear to be isolated. Based on observationsobtained at the European Southern Observatory, La Silla, Chile
| UBV polarimetry of 361 A- and F-type stars in selected areas
We present simultaneous UBV linear polarization measurements for 361 A-and F-type stars with accurate colour excess and distance determination.These stars are distributed in 35 Kapteyn's Selected Areas, covering thethird and fourth quadrants of the galactic plane (|b| <= 30degr ).The obtained polarization and the known colour excess are compared. Ananalysis of the polarization distribution as a function of the stellardistance is also performed. Based on observations collected at theEuropean Southern Observatory (ESO), La Silla, Chile.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
|
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:
|
