SEARCH · Engineering Papers
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Progress in determining the gravitational energy conversion balance in accreting white dwarf stars is reported. Detailed photospheric spectra of the white dwarf stars in the brightest magnetic cataclysmic binaries, in the region where most of the energy is radiated by the stellar atmosphere - in the extreme ultraviolet band is obtained.
We identify an abundant population of extreme emission line galaxies at redshift z=1.6 - 1.8 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared, broad-band fluxes. Supported by spectroscopic confirmation of strong [OIII] emission lines - with equivalent widths approximately 1000A - in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are dwarf galaxies with approximately 10(exp 8) solar mass in stellar mass, undergoing an enormous star-burst phase with M*/M* of only approximately 10 Myr. The star formation activity and the co-moving number density (3.7 x 10(exp -4) Mpc(exp -3)) imply that strong, short-lived bursts play a significant, perhaps even dominant role in the formation and evolution of dwarf galaxies at z greater than 1. The observed star formation activity can produce in less than 5 Gyr the same amount of stellar mass density as is presently contained in dwarf galaxies. Therefore, our observations provide a strong indication that the stellar populations of present-day dwarf galaxies formed mainly in strong, short-lived bursts, mostly at z greater than 1.
Why study Brown Dwarf stars? They re the lowest mass byproducts of star formation.. They provide time capsules across the age of the Galaxy.. They show what low-T(sub eff) atmospheres look like.. They may be some of our closest neighbors in space..WISE is a 40cm Earth-orbiting telescope. There are 211 stars and only 33 brown dwarfs in this volume.. This means that stars outnumber brown dwarfs by a factor of 6:1 currently.. The number of brown dwarfs will continue to increase if:: (a) more nearby Y dwarf candidates are confirmed, or (b) our distances to known Y s are overestimated, or (c) there are colder BDs invisible to WISE..
M dwarf stars are excellent candidates around which to search for exoplanets, including temperate, Earth-sized planets. To evaluate the photochemistry of the planetary atmosphere, it is essential to characterize the UV spectral energy distribution of the planet’s host star. This wavelength regime is important because molecules in the planetary atmosphere such as oxygen and ozone have highly wavelength-dependent absorption cross sections that peak in the UV (900–3200 Å). We seek to provide a broadly applicable method of estimating the UV emission of an M dwarf, without direct UV data, by identifying a relationship between noncontemporaneous optical and UV observations. Our work uses the largest sample of M dwarf star far- and near-UV observations yet assembled. We evaluate three commonly observed optical chromospheric activity indices—Hα equivalent widths and log{sub 10} L Hα /L bol , and the Mount Wilson Ca ii H&K S and R HK ' indices—using optical spectra from the HARPS, UVES, and HIRES archives and new HIRES spectra. Archival and new Hubble Space Telescope COS and STIS spectra are used to measure line fluxes for the brightest chromospheric and transition region emission lines between 1200 and 2800 Å. Our results show a correlation between UV emission-line luminosity normalized to the stellar bolometric luminosity and Ca ii R HK ' with standard deviations of 0.31–0.61 dex (factors of ~2–4) about the best-fit lines. We also find correlations between normalized UV line luminosity and Hα log 10 L Hα /L bol and the S index. These relationships allow one to estimate the average UV emission from M0 to M9 dwarfs when UV data are not available.
Abstract We study the initiation of thermonuclear detonations in tidally disrupted white dwarf stars by intermediate-mass (10 3 M ⊙ ) black holes. The length scales required to resolve the initiation mechanism are not easily reached in 3D, so instead we have devised 2D proxy models, which, together with a logarithmic gridding strategy, can adequately capture detonation wave fronts as material undergoes simultaneous compression and stretching from tidal forces. We consider 0.15 and 0.6 M ⊙ white dwarf stars parameterized by tidal strengths in the range β = 4–23. High spatial resolution elucidates the manner and conditions leading to thermonuclear detonation, linking the initiation sequence to stellar composition and tidal strength. All of our models suffer sustained detonations triggered by a combination of adiabatic compression, mild thermonuclear preconditioning, and collisional heating, in degrees depending primarily on tidal strength. We find that many diagnostics, such as temperature, total released energy, and iron-group products, are fairly well converged (better than 10%) at resolutions below 10 km along the scale height of the orbital plane. The exceptions are intermediate-mass transients like calcium, which remain uncertain up to factors of 2, even at 1 km resolution.
The dwarf M stars, YZ Canis Minoris and AD Leonis, exhibit narrow-band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars, AD Leonis and Wolf 424, emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process. They are attributed to coherent mechanisms such as an electron-cyclotron maser or coherent plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic gyrofrequency, the coronal magnetic field strength equals 250 G or 167 G, and constraints on the plasma frequency imply an electron density of 6 x 10 to the 9th/cu cm. Radio spikes from AD Leonis and Wolf 424 have rise times less than or equal to 5 ms, indicating a linear size of less than or equal to 1.5 x 10 to the 8th cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour.
We develop a comprehensive statistical model for the distribution of observed separations in binary star systems, in terms of distributions of orbital elements, projection effects, and distances to systems. We use this model to derive several diagnostics for estimating the completeness of imaging searches for stellar companions, and the underlying stellar multiplicities. In application to recent imaging searches for low-luminosity companions to nearby M dwarf stars, and for companions to young stars in nearby star-forming regions, our analyses reveal substantial uncertainty in estimates of stellar multiplicity. For binary stars with late-type dwarf companions, semimajor axes appear to be distributed approximately as a(exp -1) for values ranging from about one to several thousand astronomical units. About one-quarter of the companions to field F and G dwarf stars have semimajor axes less than 1 AU, and about 15% lie beyond 1000 AU. The geometric efficiency (fraction of companions imaged onto the detector) of imaging searches is nearly independent of distances to program stars and orbital eccentricities, and varies only slowly with detector spatial limitations.
We use a time-dependent hydrodynamics code to follow the propagation of acoustic waves into the corona of an M dwarf star. An important qualitative difference between M dwarfs and stars such as the Sun is that the acoustic spectrum in M dwarfs is expected to peak at periods close to the acoustic cutoff P(sub A): this allows more effective penetration of waves into the corona. In our code, radiative losses in the photosphere, chromosphere, and corona are computed using Rosseland mean opacities, Mg II kappa and Ly alpha emission, and optically thin emissivities respectively. We find that acoustic heating can maintain a corona with a temperature of order 0.7-1 x 10(exp 6) K and a surface X-ray flux as large as 10(exp 5)ergs/sq cm/s. In a recent survey of X-rays from M dwarfs, some (20%-30%) of the stars lie at or below this limiting X-ray flux: we suggest that such stars may be candidates for acoustically maintained coronae.
Integrated fluxes of chromospheric and transition region emission lines have been measured from low-resolution IUE spectra, obtained between 1981 and 1985, of a large number of chromospherically active stars. It is found that chromospherically active stars of the same R(C IV) show a spread in rotation rate that is as large as that of dwarf stars at fixed R(C IV) and that is correlated with spectral type. The UV emission of the active chromospheric stars decreases in strength with declining rotation rate, but shows little dependence upon rotation within individual luminosity classes. It is suggested that the lack of differences noted between rotation-activity relations for single, slowly rotating dwarf stars is due to the monotonic increase of both the average Rossby number and the average rotation period with later spectral type along the main sequence.
In the present study of astrometric, photometric, and spectrophotometric data for the low luminosity carbon star G77-61, radial velocity variations are detected which have a binary period of 245 days. The unseen companion is probably a cool white dwarf of much higher mass than the visible object. The most straightforward evolutionary hypothesis is that this star has an extremely metal-poor composition, and that it accreted a small amount of carbon-rich material when the now-unseen primary was at maximum radius. This may have inverted the C/O abundance of the secondary without achieving common envelope evolution and a sorter period.
We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z approx. 1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines . with rest-frame equivalent widths approx. 1000A in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with approx.10(exp 8) Solar Mass in stellar mass, undergoing an enormous starburst phase with M*/M* of only approx. 15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10(exp -4) Mpc(sup -3) can produce in approx.4 Gyr much of the stellar mass density that is presently contained in 10(exp 8) - 10(exp 9) Solar Mass dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z > 1.
It is shown how to obtain interesting limits on the number of comets in orbit around other single stars in clouds similar to the one present in our own solar system. The number of comets around white dwarf stars is constrained directly, and this in turn constrains the presence of comets around main-sequence stars. If a white dwarf possesses a cloud of comets similar to the Oort cloud, it will occassionally accrete a comet. The accretion of a comet may add enough heavy elements to the atmosphere of the white dwarf to produce a recognizable spectral signature; it is suggested that the calcium reported in the white dwarf G 74-7 is due to recent comet accretion. This signature will persist until sedimentation has removed the heavy elements from the atmosphere; the mean time between accretion events is comparable to the sedimentation time for cool DA stars, so this signature should be frequently observed if cometary systems are common. The possibility that cometary systems greater than or approximately equal to 3 times more massive than the Oort cloud are present around most white dwarf stars are excluded and how this limit can be improved is indicated. It is shown that a star keeps most of its comets during the vigorous mass loss phase prior to the appearance of the white dwarf star. Thus, the constraints obtained here may be applied to the main-sequence progenitors of the white dwarfs.
The detection of 4.75 GHz radio emissions from a white dwarf star in SU UMa is reported, and the source of the emission is discussed. The emission was discovered during a survey of six dwarf stars with a double horn receiver system. SU UMa was successfully scanned 123 times, with each scan comprising 31 3-sec integrations 30 arcsec apart. Average fluxes for each beam position were calculated, as was the X ray emission of 7.6 x 10 to the 54th/cu cm in the 0.1-4.5 keV band. The small mass outflow projected for the object indicates a source of suprathermal electrons for the radio emissions A cyclotron maser instability is suggested as the mechanism, and future measurements to detect circular polarization as proof of a coherent source are indicated.
Orbital phase-dependent variations in thermal emission and reflected stellar energy spectra can provide meaningful constraints on the climate states of terrestrial extrasolar planets orbiting M dwarf stars. Spatial distributions of water vapor, clouds, and surface ice are controlled by climate. In turn, water, in each of its thermodynamic phases, imposes significant modulations to thermal and reflected planetary spectra. Here we explore these characteristic spectral signals, based on 3D climate simulations of Earth-sized aquaplanets orbiting M dwarf stars near the habitable zone. By using 3D models, we can self-consistently predict surface temperatures and the location ofwater vapor, clouds, and surface ice in the climate system. Habitable zone planets in M dwarf systems are expected to be in synchronous rotation with their host star and thus present distinct differences in emitted and reflected energy fluxes depending on the observed hemisphere. Here we illustrate that icy, temperate, and incipient runaway greenhouse climate states exhibit phase-dependent spectral signals that enable their characterization.
Microlensing by compact objects with masses between approximately 0.001 solar masses and approximately 300 solar masses will amplify the continuum emission of a quasar, without significantly changing its line emission. Thus, compact objects with masses associated with stars, subdwarfs, and stellar remnants will reduce the apparent equivalent widths of quasar emission lines. It is possible to detect this population of lenses by searching for an increase in the number of small equivalent width quasars with redshift. This increase was looked for, but not found, in quasar samples taken from the Einstein Medium Sensitivity Survey and the Steidel & Sargent absorption-line studies. Thus, Omega(sub c), the cosmological density of compact objects relative to the critical density, is less than or approximately equal to 0.1 in the mass range 0.01 solar masses-20 solar masses (for Omega less than 0.6). For any value of Omega, Omega(sub c) less than or approximately equal to 0.2 in the larger mass range 0.001 solar masses-60 solar masses, and Omega(sub c) less than 1 for 0.001 solar masses-300 solar masses. Subdwarfs, stellar objects, or their remnants (e.g., MACHOS) cannot close the universe.
The 600 A absorption feature in the spectra of cool DA white dwarf stars has been identified as a resonance broadening of Ly-alpha due to the hydrogen quasi molecule. Likewise, the 1400 A absorption feature in the spectra of cool and moderately warm DA white dwarf stars appears to be due to a Ly-alpha satellite line arising from the hydrogen ion quasi molecule. The strength of both features is gravity sensitive and therefore promises to be an excellent indicator of surface gravity.
M-dwarf stars are prime targets for exoplanet searches because of their close proximity and favorable properties for both planet detection and characterization. However, the potential habitability and atmospheric characterization of these exoplanetary systems depends critically on the history of high-energy stellar radiation from X-rays to NUV, which drive atmospheric mass loss and photochemistry in the planetary atmospheres. With the Far Ultraviolet M-dwarf Evolution Survey, we have assessed the evolution of the FUV radiation, specifically eight prominent emission lines, including Lyα, of M-dwarf stars with stellar rotation period and age. We demonstrate tight power-law correlations between the spectroscopic FUV features, and measure the intrinsic scatter of the quiescent FUV emissions. The luminosity evolution with rotation of these spectroscopic features is well-described by a broken power law, saturated for fast rotators, and decays with increasing Rossby number, with a typical power-law slope of −2, although likely shallower for Lyα. Our regression fits enable FUV emission-line luminosity estimates relative to bolometric from known rotation periods to within ∼0.3 dex, across eight distinct UV emission lines, with possible trends in the fit parameters as a function of source layer in the stellar atmosphere. Our detailed analysis of the UV luminosity evolution with age further shows that habitable-zone planets orbiting lower-mass stars experience much greater high-energy radiative exposure relative the same planets orbiting more massive hosts. Around early- to mid-M dwarfs, these exoplanets, at field ages, accumulate up to 10–20× more EUV energy, relative to modern Earth. Moreover, the bulk of this UV exposure likely takes place within the first Gyr of the stellar lifetime.
Some Be stars which are intermittent X-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be + WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 100 A. Either the detection or the nondetection of Be + WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion.