Spectra Of Dwarfs Research Articles

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey

Context. We present a detailed spectroscopic analysis of the stars with helium-dominated spectra in the ESO Supernova la Progenitor Survey (SPY). Aims. Atmospheric parameters, masses, and abundances of trace hydrogen are determined and discussed in the context of spectral evolution of white dwarfs. Methods. The spectra are compared with theoretical model atmospheres using a Χ 2 fitting technique, leading to determinations of effective temperature, surface gravity, and hydrogen abundance. Results. Our final sample contains 71 objects, of which 6 are new detections and 14 are reclassified from DB to DBA because of the presence of H lines. One is a cool DO with weak He II lines, 2 are composite DB+dM. 55% of the DB sample show hydrogen and are thus DBA, a significantly higher fraction than found before. Conclusions. The large incidence of DBA, and the derived total hydrogen masses are compatible with the scenario that DBs reappear around 30000 K from the DB gap by mixing and diluting a thin hydrogen layer of the order of 10 -15 M ⊙ . This hydrogen mass is then during the evolution continuously increased by interstellar accretion. There are indications that the accretion rate increases smoothly with age or decreasing temperature, a trend which continuous even below the current low temperature limit (Dufour 2006). A remaining mystery is the low accretion rate of H compared to that of Ca observed in the DBZA, but a stellar wind extending down to the lowest temperatures with decreasing strength might be part of the solution.

Outflow activity in brown dwarfs

AbstractWhile numerous studies have been aimed at understanding the properties of young brown dwarfs relatively little exploration of their potential as drivers of outflows has occurred. Forbidden emission lines are important probes of outflows from young stellar objects, as they trace the shocks which form as an outflow interacts with the ambient medium of its driving source. While forbidden emission was identified in the spectra of young brown dwarfs, indicating the presence of outflows, these lines were weak and confined to the brown dwarf continuum position. Hence their origin in an outflow could not be confirmed. Our approach to this problem, is to analyse the forbidden line regions of brown dwarfs using spectro-astrometry. Spectro-astrometry is a novel technique which allows the user to recover spatial information from a spectrum beyond the limitations of the seeing of the observation. Using this technique we have found two brown dwarf outflows to date. In this chapter we outline this technique, describe our results for the brown dwarfs ρ-Oph 102 and 2MASS1207-3932 and discuss our future plans.

The NIRSPEC Brown Dwarf Spectroscopic Survey. II. High‐ResolutionJ‐Band Spectra of M, L, and T Dwarfs

We present a sequence of high-resolution (R ~ 20,000, or 15 km s^(-1)) infrared spectra of stars and brown dwarfs spanning spectral types M2.5 to T6. Observations of 16 objects were obtained using eight echelle orders to cover part of the J band from 1.165-1.323 μm with NIRSPEC on the Keck II telescope. By comparing opacity plots and line lists, over 200 weak features in the J band are identified with either FeH or H2O transitions. Absorption by FeH attains maximum strength in the mid-L dwarfs, while H_2O absorption becomes systematically stronger toward later spectral types. Narrow resolved features broaden markedly after the M to L transition. Our high-resolution spectra also reveal that the disappearance of neutral Al lines at the boundary between M and L dwarfs is remarkably abrupt, presumably because of the formation of grains. Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by condensation. The neutral potassium (K I) doublets that dominate the J band have pressure-broadened wings that continue to broaden from ~50 km s^(-1) (FWHM) at mid M to ~500 km s^(-1) at mid T. In contrast, however, the measured pseudo-equivalent widths of these same lines reach a maximum in the mid-L dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without extensive pressure-broadened wings, indicative of a lower gravity atmosphere. Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and H_2O features, confirming its status as a rapid rotator. In contrast to other late-T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence of potassium even at high resolution, which may be a metallicity effect or a result of a cooler, higher gravity atmosphere.

Resonance potassium and sodium lines in the spectra of ultracool dwarfs

We discuss the methodological problems and results of computations of the spectral energy distributions (SEDs) of L dwarfs. Over a wide wavelength interval (λλ4000–10 000 A), the spectra of these stars are determined to a considerable extent by absorption in resonance lines of potassium (7666.961, 7701.031 A) and sodium (5891.518, 5897.489 A). We compute the extended wings of these lines using the theory of quasi-stationary broadening. We compute the cores and nearby wings (up to Δλ = 40 A from the line center) of the KI and NaI lines in a collisional approximation (van der Waals theory). In our modeling of the SED of the ultracool dwarf 2MASS J15232263+3014562 (L8), we find that the observations agree best with the COND atmospheric models of Allard et al. with T eff = 2200 K and log g = 6.0.

Hydrogen and helium line formation in OB dwarfs and giants

Aims. Hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars. Hybrid non-LTE line-formation calculations for these elements have not been discussed thoroughly so far, despite their wide use for analyses of metal line spectra. We compute synthetic spectra based on a hybrid non-LTE approach in order to test the ability of these models to reproduce high-resolution and high-S/N spectra of dwarf and giant stars and also to compare them with published grids of non-LTE (OSTAR2002) and LTE (Padova) models. Methods. Our approach solves the restricted non-LTE problem based on classical line-blanketed LTE model atmospheres. State-of-the-art model atoms and line-broadening theories are employed to model the H and He iii spectra over the entire optical range and in the near-IR. Results. A comparison with published line-blanketed non-LTE models validates the suitability of the LTE approximation for modelling the atmospheric structure of late O to early B-type dwarf and giant stars at metallicities down to (at least) 1/5solar. Our hybrid non-LTE synthetic spectra simultaneously match almost all measurable hydrogen and helium lines observed in six test stars over a wide spectral range from the Balmer limit to the near-IR, except for only a few well-understood cases. A robust starting point for studies of the metal spectra is thus established. Our approach reproduces other published non-LTE calculations, but avoids inconsistencies in the modelling of the He i singlets. These have recently been discussed in the literature in the context of O-type stars and we show that they persist in the early B-types. Our approach improves on published pure LTE models – widely applied for OB star analyses – in many aspects: non-LTE strengthening and the use of improved line-broadening data result in significant differences in the line profiles and equivalent widths of the Balmer and helium lines. Where possible, systematic effects on the stellar parameter determination are quantified, e.g. gravities derived from the Hγ wings may be overestimated by up to ~0.2 dex for dwarfs at our upper temperature boundary of 35 000 K in LTE.

Study of the K-H${_2}$ quasi-molecular line satellite in the potassium resonance line

Context. The optical spectra of L and T-type dwarfs exhibit a continuum dominated by the far wings of the absorption profiles of the Na 3s-3p and K 4s-4p doublet perturbed by molecular hydrogen and helium. The presence of a broad absorption feature in the blue wing of the K 4s-4p doublet may be attributed to the K-H 2 quasi-molecular line satellite as we predicted in a previous work. Aims. The position of the predicted line satellite was not in good agreement with the observed feature. This was a motivation to compute new interaction potentials and corresponding profiles. Methods. New molecular potentials for K-H 2 were computed using a valence pseudopotential with Gaussian orbitals on potassium, for an H 2 molecule assumed to be in its ground state. A quasi-full three-electron multireference calculation agrees very well with available spectroscopic data. The potentials and radiative dipole transition moments were input data for a unified spectral line shape evaluation of the complete K resonance line profile. Results. We show that these improved potentials now predict a line satellite at the position that is observed in late brown dwarf spectra. The existence, shape and strength of this feature is found to depend strongly on temperature, as well on the abundance ratio of potassium and sodium. Conclusions. The structure of the continuum in brown dwarf spectra from 500 to 1000 nm is determined by radiative collisions of alkali atoms with H 2 molecules. We conclude that the sensitivity of the spectrum to temperature, pressure and abundances is a tool for determining basic parameters of brown dwarf atmospheres, provided that the physics underlying its formation is described well.

The Late‐T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

(Abridged) We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5+/-0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

Spectropolarimetric Survey of Hydrogen‐rich White Dwarf Stars

We have conducted a survey of 61 southern white dwarfs searching for magnetic fields using Zeeman spectropolarimetry. Our objective is to obtain a magnetic field distribution for these objects and, in particular, to find white dwarfs with weak fields. We found one possible candidate (WD 0310-688) that may have a weak magnetic field of -6.1 ±2.2 kG. Next, we determine the fraction and distribution of magnetic white dwarfs in the solar neighborhood and investigate the probability of finding more of these objects based on the current incidence of magnetism in white dwarfs within 20 pc of the Sun. We have also analyzed the spectra of the white dwarfs to obtain effective temperatures and surface gravities.

Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J17072343-0558249AB

We present Infrared Telescope Facility SpeX observations of the M/L binary system 2MASS J17072343-0558249. SpeX imaging resolves the system into a 101 ± 017 visual binary in which both components have red near-infrared colors. Resolved low-resolution (R ~ 150) 0.8-2.5 μm spectroscopy reveals strong H2O, CO, and FeH bands and alkali lines in the spectra of both components, characteristic of late-type M and L dwarfs. A comparison to a sample of late-type field dwarf spectra indicates spectral types M9 and L3. Despite the small proper motion of the system (0100 ± 0009 yr-1), imaging observations over 2.5 yr provide strong evidence that the two components share common proper motion. Physical association is also likely due to the small spatial volume occupied by the two components (based on spectrophotometric distance estimates of 15 ± 1 pc) as compared to the relatively low spatial density of low-mass field stars. The projected separation of the system is 15 ± 3 AU, similar to other late-type M and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of the binary components to be 0.072-0.083 and 0.064-0.077 M⊙, with an orbital period of roughly 150-300 yr. While this is nominally too long a baseline for astrometric mass measurements, the proximity and relatively wide angular separation of the 2MASS J1707-0558AB pair make it an ideal system for studying the M dwarf/L dwarf transition at a fixed age and metallicity.

Establishing HZ43 A, Sirius B, and RX J185635-3754 as soft X-ray standards: a cross-calibration between the Chandra LETG+HRC-S, the EUVE spectrometer, and the ROSAT PSPC

The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. We analyze the Chandra LETG+HRC-S observations of the white dwarfs HZ43 A and Sirius B and of the neutron star RX J185635-3754 in order to resolve current uncertainties in the soft X-ray spectral fluxes and photospheric parameters of the three stars. We have obtained improved parameters for which fit the observations from the optical to the soft X-ray regime. Our approach allows us to quote their absolute spectral fluxes at selected wavelengths which may aid the calibration of other space-borne instruments.

Calibration of Synthetic Photometry Using DA White Dwarfs

We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Stromgren uvby filters, the Two Micron All Sky Survey JHKs, and the Sloan Digital Sky Survey ugriz filters. Synthetic magnitudes are calculated from model white dwarf spectra folded through the published filter response functions; these magnitudes in turn are absolutely calibrated with respect to the HST flux scale. Effective zero-magnitude fluxes and zero-point offsets of each system are determined. In order to verify the external observational consistency, as well as to demonstrate the applicability of these definitions, the synthetic magnitudes are compared with the respective observed magnitudes of larger sets of DA white dwarfs that have well-determined effective temperatures and surface gravities and span a wide range in both of these parameters.

On the relevance of Compton scattering for the soft X-ray spectra of hot DA white dwarfs

We re-examine the effects of Compton scattering on the emergent spectra of hot DA white dwarfs in the soft X-ray range. Earlier studies have implied that sensitive X-ray observations at wavelengths $\lambda < 50$ \AA might be capable of probing the flux deficits predicted by the redistribution of electron-scattered X-ray photons toward longer wavelengths. We adopt two independent numerical approaches to the inclusion of Compton scattering in the computation of pure hydrogen atmospheres in hydrostatic equilibrium. One employs the Kompaneets diffusion approximation formalism, while the other uses the cross-sections and redistribution functions of Guilbert. Models and emergent spectra are computed for stellar parameters representative of HZ 43 and Sirius B, and for models with an effective temperature $T_{\rm eff} = 100 000$ K. The differences between emergent spectra computed for Compton and Thomson scattering cases are completely negligible in the case of both HZ 43 and Sirius B models, and are also negligible for all practical purposes for models with temperatures as high as $T_{\rm eff} = 100 000$ K. Models of the soft X-ray flux from these stars are instead dominated by uncertainties in their fundamental parameters.

J-, H-, and K-band Spectra of Three SU UMa-Type Dwarf Novae

AbstractWe present the results of J-, H-, and K-band spectroscopy on three SU UMa-type dwarf novae with orbital periods of 1.33 – 1.37 hr. We performed the SED fitting for the obtained spectra by assuming a power-law distribution for the accretion disk and using template spectra of late-type dwarfs for secondary star. ASAS 002511+1217.2 and EQ J183926+260409 are WZ Sge-type or WX Cet-type dwarf novae. We found strong water absorption features in their spectra, which are characteristic in late M- or L-type dwarfs. The SED fitting suggests that their secondary contributions to the overall SED are less than one third. We identified the secondary stars as M9 and L1 type dwarfs, which are rather less massive but still normal stars. The spectrum of SDSS J013701−091235 is dominated by the secondary component. Spectral features of this object are similar to those of an early M-type dwarf in spite of its short orbital period. The spectrum of SDSS 0137 strongly suggests that the evolutionary path of this object is different from that of ordinary CVs, and this object is a candidate of EI Psc-like systems.

Collisional line profiles of rubidium and cesium perturbed by helium and molecular hydrogen

Previous calculations of the far wings of the potassium and sodium doublets in a unified-line shape semi-classical theory have been used recently to improve theoretical modeling of synthetic spectra of brown dwarfs. However the comparison with observations shows missing opacity in the 0.85 to 1.1 μ m range. Contributions of other alkalis such as Rb I and Cs I may resolve this problem. We present new theoretical calculations of the line profiles of these two alkalis perturbed by helium and molecular hydrogen.

Hubble Space TelescopeFaint Object Spectrograph Spectral Mapping of V2051 Ophiuchi in a Low State

We report a study of the spectra and structure of the accretion disk of the dwarf nova V2051 Ophiuchi while the star was in an unusual faint brightness state during 1996. The differences between the UV resonant lines and continuum disk surface brightness distributions indicate a vertically extended disk with the emission from these lines arising from the upper atmospheric layers. Distinct emission along the stream trajectory suggests the occurrence of gas stream overflow. Spatially resolved spectra show that the lines are in emission at all disk radii. The Balmer decrement becomes shallower with increasing radius. The FWHMs of the emission lines show differences with respect to that expected for a gas in Keplerian rotation, and the line intensities drop with a radial dependency of I ∝ R-1.78. The uneclipsed light contributes about 5%-10% of the total flux, and its spectrum is dominated by a Balmer jump and strong lines in emission. Broad absorption bands, possibly due to Fe II, are present in the spectra of the disk side farther away from the secondary star, suggesting it arises from absorption by a extended gas region above the disk; the differences between the spectra of the hemisphere farther from and nearer to the secondary star are interpreted in terms of chromospheric emission from a disk with a nonnegligible opening angle (limb-brightening effect). Stellar atmosphere model fits to the extracted white dwarf spectrum lead to a temperature TWD = 9500 K and a distance d = 67 pc if the inner disk is opaque or d = 92 pc if the inner disk is optically thin.

L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence with observed spectra and infrared colors for early- and mid-L dwarfs and for mid- to late-T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and, therefore, that gravity is the second parameter of the L/T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams upon various cloud properties, such as particle size and cloud spatial distribution. In the region of the L$\to$T transition, we find that no one cloud-particle-size and gravity combination can be made to fit all the observed data. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund (2002) produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L$\to$T transition region might be achieved.

Phase-resolved spectroscopy of the helium dwarf nova ‘SN 2003aw’ in quiescence

High time resolution spectroscopic observations of the ultracompact helium dwarf nova 'SN 2003aw' in its quiescent state at V similar to 20.5 reveal its orbital period at 2027.8 +/- 0.5 s or 33.80 min. Together with the photometric 'superhump' period of 2041.5 +/- 0.5 s, this implies a mass ratio q approximate to 0.036. We compare both the average and time-resolved spectra of 'SN 2003aw' and Sloan Digital Sky Survey (SDSS) J124058.03-015919.2. Both show a DB white dwarf spectrum plus an optically thin, helium-dominated accretion disc. 'SN 2003aw' distinguishes itself from the SDSS source by its strong calcium H & K emission lines, suggesting higher abundances of heavy metals than the SDSS source. The silicon and iron emission lines observed in the SDSS source are about twice as strong in 'SN 2003aw'. The peculiar 'double bright spot' accretion disc feature seen in the SDSS source is also present in time-resolved spectra of 'SN 2003aw', albeit much weaker.

Theoretical modelling of optical and IR spectra of brown dwarfs and ultracool dwarfs

The current state of theoretical modelling of the spectra of cool dwarfs and ultracool objects are discussed. Fits of synthetical spectral energy distributions (SEDs) in the optical and IR spectral regions to observed spectra show problems of theoretical modelling. Problems of interpretation of observed spectra are more obvious for the case of L and T dwarfs. Still even for late-M dwarfs the situation is far from being perfect. Some examples of the .ne analysis of the spectra of ultracool dwarfs are presented. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface imaging with atomic and molecular features

We present a new Doppler-imaging inversion code that uses quasi-optimal filtering of the object's principal components of a Fisher information matrix. The new code allows one to perform the reconstruction of stellar surface temperature maps using molecular features like TiO, CO, OH, CN etc. which are numerous in spectra of late-type dwarfs. It is possible to use atomic and molecular features simultaneously in the restoration. Our tests verify the robustness of OPC but also emphasize its dependency on the various atomic and molecular line lists. Without inclusion of molecules, Doppler imaging of stars with photospheric temperatures below ≈ 4250 K (K7-M0) remains unreliable.

Theoretical profiles of light alkali resonance lines for brown dwarf atmosphere conditions

The analysis of the far wings of resonance lines of alkali elements in brown dwarf spectra requires their accurate determination. A unified theory of collisional line profiles has been applied for the evaluation of absorption profiles of alkalis perturbed by helium and molecular hydrogen. The study of the dependence on temperature of the far wings of Li-He and Li-H2 collisional profiles is reported.