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The Evolution of Cataclysmic Variables as Revealed by Their Donor Stars
We present an attempt to reconstruct the complete evolutionary pathfollowed by cataclysmic variables (CVs), based on the observedmass-radius relationship of their donor stars. Along the way, we updatethe semi-empirical CV donor sequence presented previously by one of us,present a comprehensive review of the connection between CV evolutionand the secondary stars in these systems, and reexamine most of thecommonly used magnetic braking (MB) recipes, finding that evenconceptually similar ones can differ greatly in both magnitude andfunctional form. The great advantage of using donor radii to infermass-transfer and angular-momentum-loss (AML) rates is that they samplethe longest accessible timescales and are most likely to represent thetrue secular (evolutionary average) rates. We show explicitly that ifCVs exhibit long-term mass-transfer-rate fluctuations, as is oftenassumed, the expected variability timescales are so long that othertracers of the mass-transfer rate—including white dwarf (WD)temperatures—become unreliable. We carefully explore how much ofthe radius difference between CV donors and models of isolatedmain-sequence stars may be due to mechanisms other than mass loss. Thetidal and rotational deformation of Roche-lobe-filling stars produces ~=4.5% radius inflation below the period gap and ~= 7.9% above. Acomparison of stellar models to mass-radius data for non-interactingstars suggests a real offset of ~= 1.5% for fully convective stars(i.e., donors below the gap) and ~= 4.9% for partially radiative ones(donors above the gap). We also show that donor bloating due toirradiation is probably smaller than, and at most comparable to, theseeffects. After calibrating our models to account for these issues, wefit self-consistent evolution sequences to our compilation of donormasses and radii. In the standard model of CV evolution, AMLs below theperiod gap are assumed to be driven solely by gravitational radiation(GR), while AMLs above the gap are usually described by an MB law firstsuggested by Rappaport et al. We adopt simple scaled versions of theseAML recipes and find that these are able to match the data quite well.The optimal scaling factors turn out to be f GR = 2.47± 0.22 below the gap and f MB = 0.66 ± 0.05above (the errors here are purely statistical, and the standard modelcorresponds to f GR = f MB = 1). This revisedmodel describes the mass-radius data significantly better than thestandard model. Some of the most important implications and applicationsof our results are as follows. (1) The revised evolution sequence yieldscorrect locations for the minimum period and the upper edge of theperiod gap; the standard sequence does not. (2) The observed spectraltypes of CV donors are compatible with both standard and revised models.(3) A direct comparison of predicted and observed WD temperaturessuggests an even higher value for f GR, but this comparisonis sensitive to the assumed mean WD mass and the possible existence ofmass-transfer-rate fluctuations. (4) The predicted absolute magnitudesof donor stars in the near-infrared form a lower envelope around theobserved absolute magnitudes for systems with parallax distances. Thisis true for all of our sequences, so any of them can be used to set firmlower limits on (or obtain rough estimates of) the distances toward CVsbased only on P orb and single epoch near-IR measurements.(5) Both standard and revised sequences predict that short-period CVsshould be susceptible to dwarf nova (DN) eruptions, consistent withobservations. However, both sequences also predict that the fraction ofDNe among long-period CVs should decline with P orb above theperiod gap. Observations suggest the opposite behavior, and we discussthe possible explanations for this discrepancy. (6) Approximate orbitalperiod distributions constructed from our evolution sequences suggestthat the ratio of long-period CVs to short-period, pre-bounce CVs isabout 3 × higher for the revised sequence than the standard one.This may resolve a long-standing problem in CV evolution. Tablesdescribing our donor and evolution sequences are provided inelectronically readable form.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| The triple system HIP 96515: a low-mass eclipsing binary with a DB white dwarf companion
Context: HIP 96515 A is a double-lined spectroscopic binary included inthe SACY catalog as a potential young star. It has a visual companion(CCDM 19371-5134 B, HIP 96515 B) at 8.6 arcsec. If bound to the primary,the optical and infrared colors of this wide companion are consistentwith those of a white dwarf. Aims: We attempt to characterize thesystem HIP 96515 A&B by studying each of its components. Methods: We analyzed spectroscopic and photometric observations of HIP96515 A and its visual companion, HIP 96515 B. To confirm the system asa common proper-motion pair, we analyzed the astrometry of thecomponents using high-angular resolution infrared observations obtainedwithin a time span of two years, and archival astrometry. Results: The high-resolution optical spectrum of HIP 96515 A was usedto derive a mass ratio, M_2/M_1, close to 0.9. The optical lightcurve ofHIP 96515 A shows periodic variations with P_orbital = 2.3456 days,revealing that HIP 96515 A is an eclipsing binary with preliminaryorbital parameters of i = 89.0° ± 0.2°, and M1= 0.59 ± 0.03 M&sun; and M2 = 0.54 ±0.03 M&sun;, for the primary and secondary, respectively, atan estimated distance of 42 ± 3 pc. This is a new eclipsingbinary with component masses below 0.6 M&sun;. Multi-epochobservations of HIP 96515 A&B show that the system is a commonproper-motion pair. The optical spectrum of HIP 96515 B is consistentwith a pure helium atmosphere (DB) white dwarf. The comparison withevolutionary cooling sequence models provides Teff,WD = 19126 ± 195 K, log gWD = 8.08,MWD/M&sun; = 0.6, and a distance of ~46 pc. Theestimated WD cooling age is ~100 Myr and the total age of the object(including the main-sequence phase) is ~400 Myr. Finally, if HIP 96515A&B are coeval, and assuming a common age of ~400 Myr, thecomparison of the masses of the eclipsing binary members withevolutionary tracks shows that they are underestimated by ~15% and 10%,for the primary and secondary, respectively.Based on observations collected at the Paranal Observatory underprograms 77.C-0483(A) and 81.C-0826(A).
| Coronal Activity from the ASAS Eclipsing Binaries
We combine the catalog of eclipsing binaries from the All Sky AutomatedSurvey (ASAS) with the ROSAT All Sky Survey (RASS). The combinationresults in 836 eclipsing binaries that display coronal activity and isthe largest sample of active binary stars assembled to date. By usingthe (V-I) colors of the ASAS eclipsing binary catalog, we are able todetermine the distances and thus bolometric luminosities for themajority of eclipsing binaries that display significant stellaractivity. A typical value for the ratio of soft X-ray to bolometricluminosity is LX/Lbol? a few×10-4, similar to the ratio of soft X-ray to bolometricflux FX/Fbol in the most active regions of theSun. Unlike rapidly rotating isolated late-type dwarfs - stars withsignificant outer convection zones - a tight correlation between Rossbynumber and activity of eclipsing binaries is absent. We find evidencefor the saturation effect and marginal evidence for the so-called``super-saturation'' phenomena. Our work shows that wide-field stellarvariability searches can produce a high yield of binary stars withstrong coronal activity.The combined ASAS and RASS catalog, as well as the results of this workare available for download.
| Keck DEIMOS Spectroscopy of a GALEX UV-Selected Sample from the Medium Imaging Survey
We report results from a pilot program to obtain spectroscopy forobjects detected in the Galaxy Evolution Explorer (GALEX) Medium ImagingSurvey (MIS). Our study examines the properties of galaxies detected byGALEX fainter than the Sloan Digital Sky Survey (SDSS) spectroscopicsurvey. This is the first study to extend the techniques of Salim andcoworkers to estimate stellar masses, star formation rates (SFRs), andthe b (star formation history) parameter for star-forming galaxies outto z~0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV=23.9(AB mag) having counterparts in the SDSS Data Release 4 (DR4). Of oursample, 43 are star-forming galaxies with z<0.7, 3 have emission-lineratios indicative of active galactic nuclei with z<0.7, and 4 objectswith z>1 are QSOs, 3 of which are not previously cataloged. Wecompare our sample to a much larger sample of ~50,000 matched GALEX/SDSSgalaxies with SDSS spectroscopy; while our survey is shallow, theoptical counterparts to our sources reach ~3 mag fainter in SDSS r thanthe SDSS spectroscopic sample. We use emission-line diagnostics for thegalaxies to determine that the sample contains mostly star-forminggalaxies. The galaxies in the sample populate the blue sequence in theNUV-r versus Mr color-magnitude diagram. The derived stellarmasses of the galaxies range from 108 to 1011Msolar, and derived SFRs are between 10-1 and102 Msolar yr-1. Our sample has SFRs,luminosities, and velocity dispersions that are similar to the samplesof faint compact blue galaxies studied previously in the same redshiftrange by Koo and collaborators, Guzmán and collaborators, andPhillips and collaborators. However, our sample is ~2 mag fainter insurface brightness than the compact blue galaxies. We find that the starformation histories for a majority of the galaxies are consistent with arecent starburst within the last 100 Myr.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
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| Takýmyýldýz: | Durbun |
| Sað Açýklýk: | 19h37m08.81s |
| Yükselim: | -51°34'01.1" |
| Görünürdeki Parlaklýk: | 11.622 |
| özdevim Sað Açýklýk: | 91.7 |
| özdevim Yükselim: | -27.5 |
| B-T magnitude: | 14.678 |
| V-T magnitude: | 11.875 |
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