Search

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk., Comment: Accepted for Astrophysical Journal. 16 pages (includes 3 figures)

We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99 deg in right ascension from RA = 20.7 h to 3.3 h and spanning 2.52 deg in declination from Dec = -1.26 to 1.26 deg, for a total sky area of ~249 sq deg. Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zeropoints, achieving ~20 mmag and ~30 mmag root-mean-square (RMS) accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ~32 mas and ~35 mas RMS accuracy down to 18 mag for point sources and extended sources, respectively. For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher-level catalogue. On the photometric side, these include mean exponential and PSF magnitudes along with uncertainties, RMS scatter, chi^2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and chi^2 per degree of freedom. The here presented light-motion curve catalogue is complete down to r~21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available., Comment: MNRAS accepted

Propagating star formation in a collisional ring galaxy Arp10 is investigated by a complex approach, which includes the broad- and narrow-band photometry, long- slit spectroscopy, and scanning Fabry-Perot spectroscopy. The ionized gas velocity field obtained with best spatial resolution to date indicates a non- isotropic expansion of the outer ring with a maximum velocity 110km/s. Strong vertical and non-circular motions are also seen in the vicinity of the inner ring. Our kinematic data suggest that Arp10 has a small inclination i=22\degr and high total (luminous plus dark matter) mass of about $10^{12} M_{\odot}$ within a 50 kpc radius.The abundance of oxygen 12 + log(O/H) in both star- forming rings is about 8.6.The analysis of spectral indices provides an estimate on the propagation velocities of both rings and metallicity of the pre-collision stellar population.A small "knot" near the nucleus of Arp10,which was previously suspected as a possible candidate for collision, is now unambiguously identified as the "intruder" of at least 1/4 of the total mass of Arp~10.We use a simplified two-dimensional hydrodynamic modeling of galaxy collisions to test a collisional origin of Arp10. We confirm that the sizes of the inner and outer rings, maximum expansion velocity of the outer ring, and radial profile of the gas circular velocity can be reproduced by a near-central collision with the intruder galaxy, which occurred approximately 85Myr ago. We acknowledge that an apparent crescent- shaped distribution of H$\alpha$ emission in the outer ring is caused by a star formation threshold in the gas disk of Arp10., Comment: 17 pages, 17 figures, accepted by ApJ

We analyze the J, H, and Ks 2MASS images of 140 late-type edge-on galaxies selected from the RFGC catalog (which contains flat galaxies with major-to-minor axis ratio a/b > 7). The NIR scalelengths (h) and scaleheights (z0) of the stellar disks are determined for all selected galaxies. The mean relative ratios of the scaleheights of their stellar disks are 1.00:0.91:0.86 in J:H:Ks bands, respectively. We infer that the scaleheight determined from the Ks-band images is, on average, 13% larger than the extinction-free scaleheight. This difference is much larger if the scaleheights were found from the optical-band images. The relative thickness (z0/h) of the stellar disks correlates well with their deprojected central surface brightness obtained from the 2MASS images. This project was partially supported by grant RFBR 04-02-16518., Comment: 3 pages, 3 figures. Presented as a poster at the 207th AAS Meeting, January 8-12, 2006, Washington DC, USA

We use long-slit spectra obtained with the 6-m telescope BTA (Russian Academy of Sciences) to investigate the history of star formation in the peculiar ring galaxy Arp 10. The radial distributions of quasi-Lick spectral indices are calculated from the observed spectra and are compared with the model spectra, the latter are generated using the Starburst99 population synthesis code. Our model includes an outward propagating density wave which triggers star formation in the gas disk, and an old stellar disk. The metallicity of both old and young stellar populations is a function of radius. We show that a mix of the young and old populations is required to explain the radial distribution of the spectral indices. The density wave propagates outward with a moderate speed (of order 40 km/s), and the metallicities of both young and old populations decrease with the radius. The model indices corresponding to the alpha-elements require somewhat higher metallicities as compared to the Fe-peak elements. We acknowledge partial support from grant RFBR 04-02-16518., Comment: The poster was presented at the 207th AAS Meeting

We compare variability of radial velocity (RV) and brightness for a sample of red giants which were selected as candidates for the Grid Giant Star Survey (GGSS). The purpose of the survey is to select astrometrically stable stars as a reference frame for the future Space Interferometry Mission. We incorporate photometric data taken by ROTSE-I and being taken by ROTSE-III robotic telescopes. Typically 2/3 of all GGSS stars to the north of DEC=-35 were observed 76 and 8 times by ROTSE-I and III respectively in white-light filters. It is shown that photometric stability at the 0.02 mag level cannot be used for a pre-selection of RV-stable stars, and the most important tool remains high-resolution spectroscopy. We deeply thank the ROTSE collaboration providing us observing time at the ROTSE-IIIb telescope. We gratefully acknowledge NASA grant NRA-99-04-OSS-058., Comment: The poster was presented at 205 AAS meeting (San Diego, CA, USA). 6 pages, 3 figures

We present results of Fabry-Perot and long-slit spectroscopy of the peculiar galaxy Arp 10. The ionized gas velocity field shows evidence for significant radial motions in both outer and inner galactic rings. Long-slit spectroscopy reveals gradients of age and metallicity of stellar population in agreement with the propagating nature of star formation in the galaxy. We present strong evidence that a small ``knot'' at 5 arcsec from the center of Arp 10 is its dwarf elliptic satellite, the most probable ``intruder'' responsible for triggering the expanding rings in Arp 10., Comment: 6 pages, 5 figures, poster contribution on American Astronomical Society Meeting 205, #26.05

New sensitive Very Large Array 20 cm continuum observations of the Cartwheel, the prototypical collisional ring galaxy, were carried out with the principal aim of tracing supernova remnants that are expected to lie in the wake of the expanding ring and in the ring itself. We detect predominantly non-thermal radio continuum emission from regions associated with 13 ring HII complexes. The emission interior to the ring is confined to structures that resemble spokes of the wheel. The spokes start near bright HII complexes, and extend to around 6 arcsec (4 kpc) inward in the direction of the geometrical center of the ring. There is no apparent positional coincidence between the radio continuum and optical spokes. Radial distribution of intensity along the spokes suggests that the past star formation rate (SFR) in the Cartwheel was much lower than the current SFR. New Halpha observations were used to revise the current SFR in the Cartwheel. The revised value is 18 Msun/yr, which is a factor of 4 lower than the value reported previously, but is in good agreement with the SFR estimated from far infrared luminosity. About 30% of the observed 20 cm continuum non-thermal emission seems to originate in processes that are not related to star formation. Revised SFR in the Cartwheel is comparable to that in the rest of the ring galaxies., Comment: 6 pages, uses emulateapj.cls. Scheduled to appear in ApJL February 10, 2005

We present results of numerical N-body simulations of a galactic stellar disk embedded into a spherical dark halo. The non-linear dynamics of bending instabilities developing in the disk is studied. The bending modes, axisymmetric and not, are considered as main factors increasing the disk thickness. The relation between the disk vertical scale height and the halo+bulge-to-disk mass ratio is inferred. The method of estimation of the spherical-to-disk mass ratio for edge-on spiral galaxies based on this relation is studied and applied to constrain the spherical subsystem mass and the mass of dark halos (M_h/M_d) in seven edge-on galaxies. The values of M_h/M_d are of order 1 for our galaxies., Comment: 17 pages including 20 PostScript figures. Oral talk presented in International Workshop "Progress in Study of Astrophysical Disks: Collective and Stochastic Phenomena and Computational Tools", Volgograd, September 9-11, 2003

The ROTSE-IIIa telescope and the SSO-40 inch telescope, both located at Siding Spring Observatory, imaged the early time afterglow of GRB 030418. In this report we present observations of the early afterglow, first detected by the ROTSE-IIIa telescope 211 s after the start of the burst, and only 76 s after the end of the gamma-ray activity. We detect optical emission that rises for ~600 s, slowly varies around R=17.3 mag for ~1400 s, and then fades as a power law of index alpha=-1.36. Additionally, the ROTSE-IIIb telescope, located at McDonald Observatory, imaged the early time afterglow of GRB 030723. The behavior of this light curve was qualitatively similar to that of GRB 030418, but two magnitudes dimmer. These two afterglows are dissimilar to other afterglows such as GRB 990123 and GRB 021211. We investigate whether the early afterglow can be attributed to a synchrotron break in a cooling synchrotron spectrum as it passes through the optical band, but find this model is unable to accurately describe the early light curve. We present a simple model for gamma-ray burst emission emerging from a wind medium surrounding a massive progenitor star. This model provides an effective description of the data, and suggests that the rise of the afterglow can be ascribed to extinction in the local circumburst environment. In this interpretation, these events provide further evidence for the connection between gamma-ray bursts and the collapse of massive stars., Comment: Five pages, four figures, uses emulateapj class file. Accepted to Astrophysical Journal Letters

Using two identical telescopes at widely separated longitudes, the ROTSE-III network observed decaying emission from the remarkably bright afterglow of GRB 030329. In this report we present observations covering 56% of the period from 1.5-47 hours after the burst. We find that the light curve is piecewise consistent with a powerlaw decay. When the ROTSE-III data are combined with data reported by other groups, there is evidence for five breaks within the first 20 hours after the burst. Between two of those breaks, observations from 15.9-17.1 h after the burst at 1-s time resolution with McDonald Observatory's 2.1-m telescope reveal no evidence for fluctuations or deviations from a simple power law. Multiple breaks may indicate complex structure in the jet. There are also two unambiguous episodes at 23 and 45 hours after the burst where the intensity becomes consistent with a constant for several hours, perhaps indicating multiple injections of energy into the GRB/afterglow system., Comment: Four pages, two figures, uses emulateapj class file. Accepted to Astrophysical Journal Letters

We present results of numerical modeling made for the galactic stellar disk embedded in the spherical halo. The non-linear dynamics of bending instabilities developed in the disk is studied. The axisymmetrical bending mode is considered as a main factor increasing the disk thickness. The model dependence between the disk thickness and the halo to disk mass ratio $M_h/M_d$ is inferred. The modeling results enable us to estimate the dark halo mass for several edge-on galaxies. The obtained values of $M_h/M_d$ for the galaxies considered are of order 1 - 2., Comment: 6 pages, 2 postscript figures. The poster was presented at the AAS meeting 202 (2003)

The propagation velocity of the first gas ring in collisional ring galaxies, i.e. the velocity at which the maximum in the radial gas density profile propagates radially in the galactic disk, is usually inferred from the radial expansion velocity of gas in the first ring. Our numerical hydrodynamics modeling of ring galaxy formation however shows that the maximum radial expansion velocity of gas in the first ring ($v_{gas}$) is invariably below the propagation velocity of the first gas ring itself ($v_{ring}$). Modeling of the Cartwheel galaxy indicates that the outer ring is currently propagating at $v_{ring} \approx$ 100 km/s, while the maximum radial expansion velocity of gas in the outer ring is currently $v_{gas} \approx$ 65 km/s. Modeling of the radial B-V/V-K color gradients of the Cartwheel ring galaxy also indicates that the outer ring is propagating at $v_{ring} \ge $ 90 km/s. We show that a combined effect of inclination, finite thickness, and warping of the Cartwheel's disk might be responsible for the lack of angular difference in the peak positions found for the azimuthally averaged $H\alpha$, K and B surface brightness profiles of the Cartwheel's outer ring. Indeed, the radial $H\alpha$ surface brightness profiles obtained along the Cartwheel's major axis, where effects of inclination and finite thickness are minimized, do peak exterior to those at K- and B-bands. The angular difference in peak positions implies $v_{ring}$ = 110 km/s, which is in agreement with the model predictions. We briefly discuss the utility of radio continuum emission and spectral line equivalent widths for determining the propagation velocity of gas rings in collisional ring galaxies., Comment: 14 pages, 10 postscript figures. To be published in the Astronomy & Astrophysics 2003

The relationship between the geometrical properties of stellar disks (a flatness and truncation radius) and the disk kinematics are considered for edge-on galaxies. It is shown that the observed thickness of the disks and the approximate constancy of their thickness along the radius well agrees with the condition of their marginal local gravitational stability. As a consequence, those galaxies whose disks are thinner should harbor more massive dark haloes. The correlation between the de-projeced central brightness of the disks and their flatness is found (the low surface brightness disks tend to be the thinniest ones). We also show that positions of observed photometrically determined truncation radii $R_{cut}$ for the stellar disks support the idea of marginal local gravitational stability of gaseous protodisks at $R =R_{cut}$, and hence the steepening of photometric profiles may be a result of too inefficient star formation beyond $R_{cut}$., Comment: 10 pages including 6 figures. Shortened version to appear in proceedings of the JENAM-2002 meeting, Porto, Portugal, 2002

We consider a thickness of stellar disks of late-type galaxies by analyzing the R and K_s band photometric profiles for two independent samples of edge-on galaxies. The main goal is to verify a hypotesis that a thickness of old stellar disks is related to the relative masses of the spherical and disk components of galaxies. We confirm that the radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with the increasing of total mass-to-light ratio of the galaxies, which characterize the contribution of dark halo to the total mass, and with the decreasing of central deprojected disk brightness (surface density). Our results are in good agreement with numerical models of collisionless disks evolved from subcritical velocity dispersion state to a marginally stable equilibrium state. This suggests that in most galaxies the vertical stellar velocity dispersion, which determine the equilibrium disk thickness, is close to the minimum value, that ensures disk stability. The thinnest edge-on disks appear to be low brightness galaxies (after deprojection) in which a dark halo mass far exceeds a mass of the stellar disk., Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002)

In this paper we model and analyse the B-V/V-K radial color gradients observed in the Cartwheel ring galaxy. Along with the color-color diagrams, we use the Q_BVK combined color indices, which minimise the uncertainties in the observed B-V and V-K colors introduced by dust extinction. To model the optical and near-infrared color properties of the Cartwheel galaxy, we assume that an intruder galaxy generates an expanding ring density wave in the Cartwheel's disk, which in its turn triggers massive star formation along the wave's perimeter according to the Schmidt law. We use the population synthesis to calculate the color properties of stellar populations formed in the expanding density wave. The results of color modelling suggest that the pre-collision Cartwheel was a late-type spiral, embedded in an extensive gaseous disk of sub-critical surface density. The properties of the old stellar disk are typical for the late-type Freeman disks, with the central surface brightness in V-band and the scale length being 21.0 mag arcsec^-2 and 2 kpc respectively. The pre-collision gaseous disk has a metallicity gradient ranging from z=z_sun/5 at the outer regions to z=z_sun in the central regions. At present, the wave of star formation has passed the initial extent of the pre-collision, old stellar disk and is currently moving in the predominantly gaseous, low-metallicity disk at the radius of 16 kpc. Neither young stellar populations formed in an expanding density wave, nor their mixture with the old, pre-collision stellar populations can reproduce the B-V and V-K colors of the Cartwheel's nucleus+inner ring. We find that an additional 10-Myr-old burst of star formation in the nuclear regions, along with the visual extinction of 1.3 mag, might be responsible for the peculiar colors of the Cartwheel's nucleus., Comment: 11 pages, 7 figures, accepted for publication in Astronomy and Astrophysics

Photometric deprojection is used to determine the stellardisk and bulge parameters for several edgeon galaxies from the FGC catalog. The assumption that the galaxies of our sample belonging to the fourth (i.e., lowest) surfacebrightness class in the FGC are edgeon, lowsurfacebrightness (LSB) galaxies is considered., Comment: 6 pages including 3 figures. Have been published in Astronomy Letters, v.26, p.219, 2000 (Translated from Pis'ma v Astronomicheskiy Zhurnal, V.26, N.4, 2000, p.266.)

Photometric parameters of stellar disks and bulges for several edge-on galaxies from the Catalog of Flat Galaxies (FGC) were determined. We discuss a difference between photometric parameters of LSB and HSB galaxies from our sample. Also we present results of R CCD photometry of edge-on galaxy RFGC 3647. Deprojecting this galaxy we show that it has thin LSB disk (central surface brightness $22^m.2$ in R) and high ratio of radial to vertical scale lengths. It is shown that initially gaseous disk of the galaxy was unstable and its velocity dispersion was low. Stellar disk of this LSB galaxy was not heated significantly since that time., Comment: 11 pages, 5 postscript figures. Was presented as a poster at JENAM-2000 meeting (29 May - 3 June, 2000, Moscow). Some corrections has been done

We present the results of BVRI photometry of two galaxies with active star formation: NGC 834 and NGC 1134. Combined color index Q_{BVI} was used to investigate the photometrical structure of the galaxies. Index Q_{BVI} is not affected by internal extinction and is sensitive to the presence of blue stars. Ring-like region with active star formation at 15" from the center reveals itself in the Q_{BVI} map of NGC 834. Three-arm spiral structure is well-seen on the Q_{BVI} map of NGC 1134. We propose to use the combined indexes Q_{BVI} and similarly defined indices as a tracers of Star Formation activity and structure of dusty galaxies., Comment: 3 pages, 4 embedded figures, LaTeX2e, using the EslabStyle.cls file, presented as a poster in the 33rd ESLAB Symp. "Star formation from the small to the large scale", Noordwijk, The Netherlands, 2-5 November 1999, (F. Favata, A.A. Kaas & A. Wilson eds, ESA SP-445)

We used three indicators of massive star formation: $H_{\alpha}$, FIR and non-thermal radio luminosities, to compare estimations of Star Formation Rate (SFR) for the sample of 34 spiral galaxies. To adjust SFR values, obtained from different indicators, we considered the slope $\alpha$ and/or upper mass limit $M_{up}$ of Initial Mass Function (IMF) as free parameters. The best agreement between these indicators reaches for $M_{up} \approx 60 - 100 ~M_{\odot}$ and $\alpha \approx -3.1 $ in the high mass end of IMF ($M > 10 ~M_{\odot}$). Parallel with SFR we also estimated FIR excess $X_{FIR}$ defined as a fraction of the observed FIR which is not related to young massive stars directly. $X_{FIR}$ is found to be well correlated with types of spiral galaxies and their colors (B-V): the redder a galaxy the higher is its FIR excess. We conclude that for any parameters of IMF the observed FIR flux of early type spiral galaxies needs the additional source of energy but the massive star radiation., Comment: 7 pages including 3 PostScript figures. To be published in "The Evolution of galaxies on Cosmological timescales", Ap. & Space Science, Proceedings of the meeting held at Puerto de la Cruz (Spain) on November 30-December 5, 1998