1907259
Spectroscopic analysis of DA white dwarfs from the McCook & Sion catalog
For some years now, we have been gathering optical spectra of DA white dwarfs in an effort to study and define the empirical ZZ Ceti instability strip. However, we have recently expanded this survey to include all the DA white dwarfs in the McCook & Sion catalog down to a limiting visual magnitude of V = 17.5. We present here a spectroscopic analysis of over 1000 DA white dwarfs from this ongoing survey. We have several specific areas of interest most notably the hot DAO white dwarfs, the ZZ Ceti instability strip, and the DA+dM binary systems. Furthermore, we present a comparison of the ensemble properties of our sample with those of other large surveys of DA white dwarfs, paying particular attention to the distribution of mass as a function of effective temperature.
18172579
Evolution of the nuclear accretion disk emission in NGC 1097 : getting closer to the black hole
We study the evolution of the broad, double-peaked Hα emission-line profile of the LINER/Seyfert 1 nucleus of NGC 1097, using 24 spectra obtained over a time span of 11 years—from 1991 November through 2002 October. While in the first 5 years the main variation was in the relative intensity of the blue and red peaks, in the last years we have also observed an increasing separation between the two peaks, at the same time as the integrated flux in the broad line has decreased. We propose a scenario in which the emission originates in an asymmetric accretion disk around a supermassive black hole, whose source of ionization is getting dimmer, causing the region of maximum emission to come closer to the center (and thus to regions of higher projected velocity). We use the observations to constrain the evolution of the accretion disk emission and to evaluate two models: the elliptical-disk model previously found to reproduce the observations from 1991 to 1996 and a model of a circular disk with a single spiral arm. In both models the peak emissivity of the disk drifts inward with time, while the azimuthal orientation of the elliptical-disk or the spiral pattern varies with time. In the case of the spiral-arm model, the whole set of data is consistent with a monotonic precession of the spiral pattern, which has completed almost two revolutions since 1991. Thus, we favor the spiral-arm model, which, through the precession period, implies a black hole mass that is consistent with the observed stellar velocity dispersion. In contrast, the elliptical-disk model requires a mass that is an order of magnitude lower. Finally, we have found tentative evidence of the emergence of an accretion disk wind, which we hope to explore further with future observations.
17086551
Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the Local Group irregular galaxy NGC 6822. The data consist of VLT FORS observations in the 3450-7500 A range. We have obtained electron temperatures and densities using different line intensity ratios. We have derived the He, C, and O abundances relative to H based on recombination lines; the abundance ratios among these elements are almost independent of the temperature structure of the nebulae. We have also determined the N, O, Ne, S, Cl, and Ar abundances based on collisionally excited lines; the ratios of these abundances relative to that of H depend strongly on the temperature structure of the nebulae. The chemical composition of NGC 6822 V is compared with those of the Sun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC. The value of O/H derived from recombination lines is in good agreement with that derived by Venn and coworkers from two A-type supergiants in NGC 6822.
118864206
Search for giant planets in M 67 - I. Overview
Context. Precise stellar radial velocities (RVs) are used to search for massive (Jupiter masses or higher) exoplanets around the stars of the open cluster M 67. Aims. We aim to obtain a census of massive exoplanets in a cluster of solar metallicity and age in order to study the dependence of planet formation on stellar mass and to compare in detail the chemical composition of stars with and without planets. This first work presents the sample and the observations, discusses the cluster characteristics and the RV distribution of the stars, and individuates the most likely planetary host candidates. Methods. We observed a total of 88 main-sequence stars, subgiants, and giants all highly probable members of M 67, using four telescopes and instrument combinations: the HARPS spectrograph at the ESO 3.6 m, the SOPHIE spectrograph at OHP, the CORALIE spectrograph at the Euler swiss telescope and the HRS spectrograph at Hobby Eberly Telescope. We investigate whether exoplanets are present by obtaining RVs with precisions as good as � 10 m s −1 . To date, we have performed 680 single observations (Dec. 2011) and a preliminary analysis of data, spanning a period of up to eight years. We computed zero-point deviations for each spectrograph with respect to HARPS, finding that for SOPHIE and CORALIE the offsets are minimal (at −11.4 m s −1 and 26.8 m s −1 , respectively), while for our HET measurements the offset is larger, 242.0 m s −1 . After reducing all the observations to the HARPS zero point, the RV measurements for each star are used to evaluate the RV variability along the cluster color magnitude diagram (CMD). Results. Although the sample was pre-selected to avoid the inclusion of binaries, we identify 11 previously unknown binary candidates. The RV variance (including the observational error) for the bulk of stars is almost constant with stellar magnitude (therefore stellar gravity) at σ = 20 m s −1 . This number includes both the stellar intrinsic variability and the observational error, which is the major source of uncertainty for the faintest stars. Eleven stars clearly displayed larger RV variability and these are candidates to host long-term substellar companions. The average RV is also independent of the stellar magnitude and evolutionary status, confirming that the difference in gravitational redshift between giants and dwarfs is almost cancelled by the atmospheric motions. We use the subsample of solar-type stars to derive a precise true RV for this cluster, and we use asteroid observations to derive the zero point of the HARPS G star mask of 94.5 m s −1 . The true RV of the cluster is M 67RV = 33.74 ± 0.12 km s −1 . The velocity dispersion is 0.54 km s −1 for giants and 0.68 km s −1 for dwarfs, which in both cases is substantially lower than reported in previous works. The higher velocity dispersion of the lower mass stars is confirmed by these observations, in which for the first time the RV measurement precision is much smaller than the cluster dispersion. We finally create a catalog of binaries and use it to clean the CMD. Isochrone fitting confirms an age of around 4 Gyr. Further cleaning of the CMD based on precise RV could establish M 67 as a real benchmark for stellar evolutionary models. Conclusions. By pushing the search for planets to the faintest possible magnitudes, it is possible to observe solar analogs in open clusters, and we propose 11 candidates to host substellar companions. We also show that precise RV measurements can be used for purposes in addition to planet searches.
118914948
Optical and near-infrared observations of the peculiar type ia supernova 1999ac
We present 39 nights of optical photometry, 34 nights of IR photometry, and 4 nights of optical spectroscopy of the Type Ia supernova 1999ac. This supernova was discovered 2 weeks before maximum light, and observations were begun shortly thereafter. At early times its spectra resembled the unusual SN 1999aa and were characterized by very high velocities in the Ca II H and K lines but very low velocities in the Si II λ6355 line. The optical photometry showed a slow rise to peak brightness but, quite peculiarly, was followed by a more rapid decline from maximum. Thus, the B- and V-band light curves cannot be characterized by a single stretch factor. We argue that the best measure of the nature of this object is not the decline rate parameter Δm15(B). The B - V colors were unusual from 30 to 90 days after maximum light in that they evolved to bluer values at a much slower rate than normal Type Ia supernovae. The spectra and bolometric light curve indicate that this event was similar to the spectroscopically peculiar slow decliner SN 1999aa.
119183225
First T dwarfs in the VISTA Hemisphere Survey
School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, United KingdomOctober 19, 2012; October 19, 2012ABSTRACTAims.The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initialmass function by identifying a magnitude-limited sample ofT dwarfs in the full southern sky.Methods. We present the results of a photometric search aimed at discovering cool brown dwarfs in the Southern sky imaged atinfrared wavelengths by the Visible and Infrared Survey Telescope for Astronomy (VISTA) and the Wide Infrared Survey Explorer(WISE) satellite mission. We combined the ﬁrst data release (DR1) of the VISTA Hemisphere Survey (VHS) and the WISE prelimi-nary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool browndwarfs.Results.The VHS DR1 vs. WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previ-ously published in the literature and ﬁve new ones conﬁrmed s pectroscopically with spectral types between T4.5 and T8. We estimatethat the two T6 dwarfs lie within 16 pc and the T4.5 within 25 pc. The remaining three are 30–50 pc distant. The only T7 dwarf inour sample is the faintest of its spectral class with J =19.28 mag. The other six T dwarf candidates remain without spectroscopicfollow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch datafrom public databases (DENIS, 2MASS, VHS, WISE, Spitzer).Key words. Stars: low-mass stars and brown dwarfs — techniques: photom etric — techniques: spectroscopic — Infrared: Stars —surveys
14396422
Nuclear Properties of Nearby Spiral Galaxies from Hubble Space Telescope NICMOS Imaging and STIS Spectroscopy
We investigate the central regions of 23 spiral galaxies using Space Telescope Imaging Spectrograph (STIS) spectroscopy and archival Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging. The sample is taken from our program to determine the masses of central massive black holes (MBHs) in 54 nearby spiral galaxies. Stars are likely to contribute significantly to any dynamical central mass concentration that we find in our MBH program, and this paper is part of a series to investigate the nuclear properties of these galaxies. We use the Nuker law to fit surface brightness profiles, derived from the NICMOS images, to look for nuclear star clusters and find possible extended sources in three of the 23 galaxies studied (13%). The fact that this fraction is lower than that inferred from optical Hubble Space Telescope studies is probably due to the greater spatial resolution of those studies. Using R - H and J - H colors and equivalent widths of Hα emission (from the STIS spectra), we investigate the nature of the stellar population with evolutionary models. Under the assumption of hot stars ionizing the gas, as opposed to a weak active galactic nucleus (AGN), we find that there are young stellar populations (~10–20 Myr); however, these data do not allow us to determine what percentage of the total nuclear stellar population they form. In addition, in an attempt to find any unknown AGN, we use [N II] and [S II] line flux ratios (relative to Hα) and find tentative evidence for weak AGNs in NGC 1300 and NGC 4536.
2943456
A near‐infrared polarized bipolar cone in the Circinus galaxy
We present near–infrared broad–band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south–eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J–K and H–K colour maps reveal a complex colour gradient toward the south–east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the starformation ring. We model the polarized flux images with the scattering and torus model of Young et al. , with the same basic input parameters as used by Alexander et al. in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of �16pc, and a visual extinction AV , through the torus, to the near–infrared emission regions of >66 mags.
15205380
NICMOS imaging of molecular hydrogen emission in seyfert galaxies
We present near-infrared (NICMOS) imaging of broadband and molecular hydrogen emission in Seyfert galaxies. In six of 10 Seyfert galaxies we detect resolved or extended emission in the 1-0 S(1) 2.121 μm or 1-0 S(3) 1.9570 μm molecular hydrogen lines. We did not detect emission in the most distant galaxy or in two Seyfert 1 galaxies because of the luminosity of the nuclear point sources. In NGC 5643, NGC 2110, and Mrk 1066, molecular hydrogen emission is detected in the extended narrow-line region on scales of a few hundred parsecs from the nucleus. Emission is coincident with [O III] and Hα + [N II] line emission. This emission is also near dust lanes observed in the visible to near-infrared color maps, suggesting that a multiphase medium exists near the ionization cones and that the morphology of the line emission is dependent on the density of the ambient media. The high 1-0 S(1) or S(3) H2 to Hα flux ratio suggests that shock excitation of molecular hydrogen (rather than UV fluorescence) is the dominant excitation process in these extended features. In NGC 2992 and NGC 3227 the molecular hydrogen emission is from 800 and 100 pc diameter "disks" (respectively) that are not directly associated with [O III] emission and are near high levels of extinction (AV 10). The molecular hydrogen emission in NGC 4945 appears to be from the edge of a 100 pc superbubble. The molecular gas in these three galaxies could be excited by processes associated with local star formation. We confirm previous spectroscopic studies finding that no single mechanism is likely to be responsible for the molecular hydrogen excitation in Seyfert galaxies.
18339380
Nuclear properties of a sample of nearby spirals from STIS imaging
We present surface photometry for the central regions of a sample of 48 spiral galaxies (mostly unbarred and barred of types Sbc or Sc) observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Surface brightness profiles were derived and modeled with a Nuker law. We also analyzed archival Wide Field Planetary Camera 2 images with a larger field of view, available for 18 galaxies in our sample. We modeled the extracted bulge surface brightness profiles with an exponential, an r1/4, or an rn profile. In agreement with previous studies, we find that bulges of Sbc galaxies fall into two categories: bulges well described by an exponential profile and those well described by an r1/4 profile. Only one galaxy requires the use of a more general Sersic profile to properly describe the bulge. Nuclear photometrically distinct components are found in ∼ 55% of the galaxies. For those that we classify as star clusters based on their resolved extent we find absolute magnitudes that are brighter on average than those previously identified in spiral galaxies. This might be due to a bias in our sample toward star forming galaxies, combined with a trend for star forming galaxies to host brighter central clusters. Department of Astronomy, Univeristà degli Studi di Padova, Padova, Italy Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK Department of Physics, RIT, 84 Lomb Memorial Dr., Rochester, NY 14623-5603 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 Oxford University, Theoretical Physics, Keble Road, Oxford, OX1 3NP, UK INAF-Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy Physics Department, ETH, Hoenggerberg HPF G4.3, CH–8092 Zurich, Switzerland INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy School of Physics and Astronomy, University of Nottingham, NG7 2RD, UK Center for Astrophysical Sciences, Johns Hopkins University, 239 Bloomberg Center for Physics & Astronomy, 3400 North Charles Street, Baltimore, MD 21218
9424089
Constraints on the nuclear emission of the Circinus galaxy: the torus
In the context of the unified model of Seyfert galaxies, we use observations from the literature and a radiative transfer model to investigate the near–IR to mm emission produced by the presumed torus in the Circinus galaxy, from 2µm to 1.3mm. From the infrared SED modelling, we find that the total luminosity (LIR) in this wavelength range, consists of similar contributions from the torus and starburst with a ratio of nuclear luminosity to starburst luminosity (LNUC/LSB) � 0.8. By using a similar torus model to that of NGC1068, but without the conical dust, we find an upper limit to the outer torus radius of �12pc with a best fit of �2pc. The upper limit torus size estimated from the radiative transfer modelling is consistent with the 16pc torus radius estimated from near–IR imaging polarimetry of Circinus.