A large sample of Kohonen-selected SDSS quasars with weak emission lines: selection effects and statistical properties

We performed a search for WLQs in the spectroscopic data from the Sloan Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for nearly 10^5 quasar spectra. The final sample consists of 365 quasars and includes in particular a subsample of 46 WLQs with equivalent widths W(MgII) < 11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with those of control samples of ordinary quasars. Particular attention was paid to selection effects. The WLQs have, on average, significantly higher luminosities, Eddington ratios, and accretion rates. About half of the excess comes from a selection bias, but an intrinsic excess remains probably caused primarily by higher accretion rates. The spectral energy distribution shows a bluer continuum at rest-frame wavelengths > 1500 A. The variability in the optical and UV is relatively low, even taking the variability-luminosity anti-correlation into account. The percentage of radio detected quasars and of core-dominant radio sources is significantly higher than for the control sample, whereas the mean radio-loudness is lower. The properties of our WLQ sample can be consistently understood assuming that it consists of a mix of quasars at the beginning of a stage of increased accretion activity and of beamed radio-quiet quasars. (Abstract modified to match the arXiv format)Comment: 17 pages, 12 figures; accepted for publication in Astronomy and Astrophysics; the full catalogue is only available in electronic form at CD

Ultraviolet variability of quasars: dependence on the accretion rate

We compiled a catalogue of about 4000 SDSS quasars including individual estimators V for the variability strength, virial black hole masses M, and mass accretion rates dM/dt from the Davis-Laor scaling relation. We confirm significant anti-correlations between V and dM/dt, the Eddington ratio, and the bolometric luminosity L, respectively. A weak, statistically not significant positive trend is indicated for the dependence of V on M. As a side product, we find a strong correlation of the radiative efficiency with M and show that this trend is most likely produced by selection effects in combination with the mass errors and the use of the scaling relation for dM/dt. The anti-correlations found for V cannot be explained in such a way. The strongest anti-correlation is found with dM/dt. However, it is difficult to decide which of the quantities (L, Eddington ratio, dM/dt) is intrinsically correlated with V and which of the observed correlations are produced by the relations between these quantities. A V-dM/dt anti-correlation is qualitatively expected for the strongly inhomogeneous accretion disks. We argue that several observed variability properties are not adequately explained by the simple multi-temperature black-body model of a standard disk and suggest to check whether the strongly inhomogeneous disk model is capable of reproducing these observations better.Comment: 16 pages, 10 figures; accepted for publication in Astronomy and Astrophysics; the full catalogue is only available in electronic form at CD

A QSO survey via optical variability and zero proper motion in the M92 field.III. Narrow emission line galaxies

We study a sample of 23 narrow-emission line galaxies (NELGs) which were selected by their strong variability as QSO candidates in the framework of a variability-and-proper motion QSO survey on digitised Schmidt plates. In previous work, we have shown that variability is an efficient method to find AGNs. The variability properties of the NELGs are however significantly different from those of the QSOs. The main aim of this paper is to clarify the nature of this variability and to estimate the fraction of AGN-dominated NELGs in this sample. New photometric and spectroscopic observations are presented, along with revised data from the photographic photometry. The originally measured high variability indices could not be confirmed. The diagnostic line-ratios of the NELG spectra are consistent with HII region-like spectra. No AGN could be proved, yet we cannot rule out the existence of faint low-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

A systematic search for novae in M31 on a large set of digitized archival Schmidt plates

This paper reports on the detection of optical novae in our neighbour galaxy M31 based on digitized historical Tautenburg Schmidt plates. The accurate positions of the detected novae lead to a much larger database when searching for recurrent novae in M31. We conducted a systematic search for novae on 306 digitized Tautenburg Schmidt plates covering a time span of 36 years from 1960 to 1996. From the database of both ~ 300 000 light curves and about one million detections on only one plate per colour band, nova candidates were efficiently selected by automated algorithms and subsequently individually inspected by eye. We report the detection of 84 nova candidates. We found 55 nova candidates from the automated analysis of the light curves. Among these, 22 were previously unknown, 12 were known but not identified on Tautenburg Schmidt plates before, and 21 novae had been previously discovered on Tautenburg plates. An additional 29 known novae could be confirmed by the detailed investigation of single detections. One of our newly discovered nova candidates shows a high position coincidence with a nova detected about 30 years earlier. Therefore, this object is likely to be a recurrent nova. Furthermore, we re-investigated all 41 nova candidates previously found on Tautenburg plates and confirm all but two. Positions are given for all nova candidates with a typical accuracy of ~ 0.4 arcsec. We present light curves and finding charts as online material. The analysis of the plates has shown the wealth of information still buried in old plate archives. Extrapolating from this survey, digitization of other historical M31 plate archives (e.g. from the Mount Wilson or Asiago observatories) for a systematic nova search looks very promising.Comment: 29 pages, 14 figures, 19 tables, accepted for publication in A&A. Figs 6-14 are reduced in resolution due to the restrictions on space available on astro-ph; v2: minor grammatical change

FSR 0190: Another old distant Galactic cluster

We are conducting a large programme to classify newly discovered Milky Way star cluster candidates from Froebrich et al. Here we present near-infrared follow-up observations of FSR0190 (α = 20h05m31s.3, δ = 33°34â?²09â?², J2000). The cluster is situated close to the Galactic plane (l = 70°7302, b = +0°9498). It shows a circular shape, and a relatively large number of core helium burning stars - which clearly distinguishes the cluster from the rich field - but no centrally condensed star density profile. We derive an age of more than 7 Gyr, a Galactocentric distance of 10.5 kpc, a distance of 10 kpc from the Sun, and an extinction of AK = 0.8 mag. The estimated mass is at least of the order of 105 Mâ??, and the absolute brightness is MV â?¤ -4.7 mag; both are rather typical properties for Palomar-type globular clusters. © 2007 The Authors. Journal compilation © 2007 RAS

A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry

We continue the QSO search in the 10 square degrees Schmidt field around M92 based on variability and proper motion (VPM) constraints. We have re-reduced 162 digitised B plates with a time-baseline of more than three decades and have considerably improved both the photometric accuracy and the star-galaxy separation at B>19. QSO candidates are selected and marked with one out of three degrees of priority based on the statistical significance of their measured variability and zero proper motion. Spectroscopic follow-up observations of 84 new candidates with B>19 revealed an additional 37 QSOs and 7 Seyfert1s. In particular, all 92 high-priority candidates are spectroscopically classified now; among them are 70 QSOs and 9 Seyfert1s (success rate 86%). We expect that 87% (55%) of all QSOs with B<19.0 (19.8) are contained in this high-priority subsample. For the combined sample of high-priority and medium-priority objects, a completeness of 89% is estimated up to B_lim=19.5. The sample of all AGNs detected in the framework of the VPM search in the M92 field contains now 95 QSOs and 14 Seyfert1s with B<19.9. Although the VPM QSOs were selected by completely different criteria, their properties do not significantly differ from those of QSOs found by more traditional optical survey techniques. In particular, the spectra and the optical broad band colours do not provide any hints on a substantial population of red QSOs up to the present survey limit.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysic

High-Resolution NIR Observations of the Circumstellar Disk System in the Bok Globule CB 26

We report on results of near-infrared and optical observations of the mm disk embedded in the Bok globule CB 26 (Launhardt & Sargent 2001). The near-infrared images show a bipolar reflection nebula with a central extinction lane which coincides with the mm disk. Imaging polarimetry of this object yielded a polarization pattern which is typical for a young stellar object surrounded by a large circumstellar disk and an envelope, seen almost edge-on. The strong linear polarization in the bipolar lobes is caused by single scattering at dust grains and allowed to locate the illuminating source which coincides with the center of the mm disk. The spectral energy distribution of the YSO embedded in CB 26 resembles that of a ClassI source with a luminosity of 0.5 L_sun.Using the pre-main-sequence evolutionary tracks and the stellar mass inferred from the rotation curve of the disk, we derive an age of the system of <10^6 yr. H_alpha and [SII] narrow-band imaging as well as optical spectroscopy revealed an Herbig-Haro object 6.15 arcmin northwest of CB 26 YSO 1, perfectly aligned with the symmetry axis of the bipolar nebula. This Herbig-Haro object (HH 494) indicates ongoing accretion and outflow activity in CB 26 YSO 1. Its excitation characteristics indicate that the Herbig-Haro flow is propagating into a low-density environment. We suggest that CB 26 YSO 1 represents the transition stage between embedded protostellar accretion disks and more evolved protoplanetary disks around T Tauri stars in an undisturbed environment.Comment: 21 pages, 6 figures (reduced resolution), ApJ accepte

VPMS J1342+2840 - an unusual quasar from the variability and proper motion survey

We report the discovery of the highly peculiar, radio-loud quasar VPMS J1342+2840 (z ~ 1.3) from the variability and proper motion survey. We present spectroscopic, imaging and photometric observations. The unusual spectrum shows a strong depression of the continuum over a wide wavelength range in the blue part without the typical structures of broad absorption line (BAL) troughs. The image of the quasar is unresolved and there is no evidence for a foreground object on the line of sight. The broad-band spectral energy distribution is not consistent with obvious dust reddening with the standard SMC extinction curve. The downturn of the continuum flux of VPMS J1342+2840 at short wavelengths can be caused by dust reddening only if the reddening curve is steeper then the SMC curve in the ultraviolet and is very flat at longer wavelengths. Alternatively, the dominant spectral features can be explained by low-ionization BALs forming unusually wide, overlapping absorption troughs.Comment: 4 pages, 3 figures, accepted for publication in A&A Letter

Search for nearby stars among proper motion stars selected by optical-to-infrared photometry. I. Discovery of LHS 2090 at spectroscopic distance of d=6pc

We present the discovery of a previously unknown very nearby star - LHS 2090 at a distance of only d=6 pc. In order to find nearby (i.e. d < 25 pc) red dwarfs, we re-identified high proper motion stars ($\mu >$ 0.18 arcsec/yr) from the NLTT catalogue (Luyten \cite{luyten7980}) in optical Digitized Sky Survey data for two different epochs and in the 2MASS data base. Only proper motion stars with large $R-K_s$ colour index and with relatively bright infrared magnitudes ($K_s<10$) were selected for follow-up spectroscopy. The low-resolution spectrum of LHS 2090 and its large proper motion (0.79 arcsec/yr) classify this star as an M6.5 dwarf. The resulting spectroscopic distance estimate from comparing the infrared $JHK_s$ magnitudes of LHS 2090 with absolute magnitudes of M6.5 dwarfs is $6.0\pm1.1$ pc assuming an uncertainty in absolute magnitude of $\pm$0.4 mag.Comment: 3 pages, 1 figure, accepted for publication in Astronomy and Astrophysics Letter