TD-1 Satellite Research Articles

UV flux distributions of γ Doradus stars

Context. It seems that the recently identified class of pulsating stars, the γ Dor type-variables, includes objects with different metal abundances and a large percentage of binaries. Aims. We looked for indicators of metal abundance peculiarities and stellar binarity in a sample of 40 confirmed γ Dor stars. Methods. Absolute magnitudes from Hipparcos parallaxes and UV magnitudes, from the S2/S68 experiment on board the TD1 satellite, are retrieved from databases and compared with predicted values. A set of non variable normal stars is used to check the consistency of this analysis and also serve as reference stars. Results. Twenty-nine stars of the γ Dor star sample, which is 73% of it, are discovered having abnormal UV fluxes constantly showing UV flux excesses compared to those computed with the atmospheric parameters (Teff ,l ogg, and metallicity) determined from calibration of the uvbyβ indices. The reason for this UV excess of flux at 196.5 nm and at 236.5 nm, which was previously known only for HD 209295, cannot be ascribed to binarity alone. An extra source of UV flux or less UV absorption – yet unknown – must be present.

HR 8799: A Link between γ Doradus Variables and λ Bootis Stars

On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered γ Doradus variable star class, is a λ Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields Teff = 7430 K, log g = 4.35, microturbulent velocity ξt = 2.7 km s-1, and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a Vega-like star in that it shows excess flux at 60 μm, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the λ Bootis phenomenon and the γ Doradus pulsation phenomenon.

Convection, Thermal Bifurcation, and the Colors of A Stars

Broadband ultraviolet photometry from the TD-1 satellite and low-dispersion spectra from the short wavelength camera of IUE have been used to investigate a long-standing proposal of Bohm-Vitense that the normal main-sequence A and early-F stars may divide into two different temperature sequences: (1) a high-temperature branch (and plateau) comprised of slowly rotating convective stars, and (2) a low-temperature branch populated by rapidly rotating radiative stars. We find no evidence from either data set to support such a claim, or to confirm the existence of an A-star gap in the B-V color range 0.22 ≤ B-V ≤ 0.28 due to the sudden onset of convection. We do observe, nonetheless, a large scatter in the 1800-2000 A colors of the A-F stars, which amounts to ~0.65 mag at a given B-V color index. The scatter is not caused by interstellar or circumstellar reddening. A convincing case can also be made against binarity and intrinsic variability due to pulsations of δ Sct origin. We find no correlation with established chromospheric and coronal proxies of convection, and thus no demonstrable link to the possible onset of convection among the A-F stars. The scatter is not instrumental. Approximately 0.4 mag of the scatter is shown to arise from individual differences in surface gravity as well as a moderate spread (factor of ~3) in heavy metal abundance and UV line blanketing. A dispersion of ~0.25 mag remains, which has no clear and obvious explanation. The most likely cause, we believe, is a residual imprecision in our correction for the spread in metal abundances. However, the existing data do not rule out possible contributions from intrinsic stellar variability or from differential UV line blanketing effects owing to a dispersion in microturbulent velocity.

Powerful model for the point source sky: Far-ultraviolet and enhanced midinfrared performance

view Abstract Citations (75) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Powerful Model for the Point Source Sky: Far-Ultraviolet and Enhanced Midinfrared Performance Cohen, M. Abstract I report further developments of the Wainscoat et al. (1992) model originally created for the point source infrared sky. The already detailed and realistic representation of the Galaxy (disk, spiral arms and local spur, molecular ring, bulge, spheroid) has been improved, guided by CO surveys of local molecular clouds, and by the inclusion of a component to represent Gould's Belt. The newest version of the model is very well validated by Infrared Astronomy Satellite (IRAS) source counts. A major new aspect is the extension of the same model down to the far ultraviolet. I compare predicted and observed far-utraviolet source counts from the Apollo 16 'S201' experiment (1400 A) and the TD1 satellite (for the 1565 A band). Publication: The Astronomical Journal Pub Date: February 1994 DOI: 10.1086/116878 Bibcode: 1994AJ....107..582C Keywords: Astronomical Models; Carbon Monoxide; Far Ultraviolet Radiation; Galactic Bulge; Infrared Astronomy; Infrared Radiation; Molecular Clouds; Point Sources; Spiral Galaxies; Ultraviolet Astronomy; Apollo 16 Flight; Astronomical Spectroscopy; Infrared Astronomy Satellite; Infrared Spectra; Sky Surveys (Astronomy); Ultraviolet Spectra; Astronomy; GALAXY: STRUCTURE; GALAXY: GENERAL full text sources ADS | data products SIMBAD (9)

In-Flight Performance and Preliminary Results from the Far Ultraviolet Space Telescope (Faust) Flown on ATLAS-1

In 1992 March the Far Ultraviolet Space Telescope was flown as part of the NASA ATLAS-1 mission. We describe the in-flight instrument performance and provide a preliminary view of some of the results. Twenty-two wide-field far-ultraviolet images were obtained. At this stage of our analysis, we have far-ultraviolet photometry for more than 4800 sources; the overwhelming majority of these are below the TD-1 satellite far-ultraviolet detection limit. In these images, we detect sources to a flux limit of 1×10 -14 ergs s -1 cm -2 A -1 , which is at least an order of magnitude more sensitive than the TD-1 satellite, and we have virtually complete source detections to a level of 6×10 -14 ergs s -1 cm -2 A -1

Atlas of extinction curves derived from ultraviolet spectra of the TD-1 satellite

The collection of 166 extinction curves derived from the published low-resolution spectra acquired with the aid of the spectrometer on board the TD-1 satellite is presented. The observed variety of extinction laws is apparently due to the varied physical parameters of interstellar clouds; for example, the bright stars, included in the sample of TD-1 material, are very likely to be obscured by single clouds (interstellar or circumstellar). The system of standards (published already by the authors) constructed with the aid of a special procedure allowing the possible effects of spectral mismatch to be avoided and making possible the derivation of extinction curves even in cases of very small E (B−V) s, was applied

Vector space methods of photometric analysis. II - Refinement of the MK grid for B stars. III - The two components of ultraviolet reddening

This paper discusses systematic errors which arise from exclusive use of the MK system to determine reddening. It is found that implementation of uvby, beta photometry to refine the qualitative MK grid substantially reduces stellar mismatch error. A working definition of 'identical' ubvy, beta types is investigated and the relationship of uvby to B-V color excesses is determined. A comparison is also made of the hydrogen based uvby, beta types with the MK system based on He and metal lines. A small core correlated effective temperature luminosity error in the MK system for the early B stars is observed along with a breakdown of the MK luminosity criteria for the late B stars. The second part investigates the wavelength dependence of interstellar extinction in the ultraviolet wavelength range observed with the TD-1 satellite. In this study the sets of identical stars employed to find reddening are determined more precisely than in previous studies and consist only of normal, nonsupergiant stars. A multivariate analysis of variance techniques in an unbiased coordinate system is used for determining the wavelength dependence of reddening.

An upper limit on the EUV flux from HD192273

Cash et al.1 reported the possible detection off an extreme ultraviolet (EUV) source in the constellation Pavo. They suggested HD192273 as a possible candidate and cited a TD-1 satellite observation indicating that this star is also unexpectedly bright at 1,565 A. Wegner2 reported visual photometric and spectroscopic data for HD192273 and concluded that it is apparently a normal B-type star. Shipman and Wegner3 have reviewed the available data on HD192273 and suggested that all of the observations could be explained if this object were an analogue of HZ22, an evolved, mass-exchange, binary. We report here a recent observation with the UV spectrometer (UVS) aboard the Voyager 1 spacecraft. An upper limit (2σ) of 8 × 10−3 photon cm−2 s−1 A−1 is placed on the 534–776 A flux from HD192273 at the time of observation. This is ∼10% of the 500–780 A flux reported from Pavo by Cash et al. The 1,565 A flux observed by TD-1 is confirmed and is consistent with the stellar parameters determined by Wegner. Discounting the possibility that HD192273 is an EUV variable, it is probably not the potential source reported by Cash et al.

Observations of the central stars of planetary nebulae with the sky-survey telescope of the TD-1 satellite

Observations of the central stars of planetary nebulae with the sky-survey telescope of the TD-1 satellite

Observations of the Large Magellanic Cloud with the S2/68 ultraviolet sky-survey telescope in the TD-1 satellite

Observations of the Large Magellanic Cloud with the S2/68 ultraviolet sky-survey telescope in the TD-1 satellite

ABSOLUTE CALIBRATION IN THE 1750 Å - 3350 Å REGION.

ABSTRACT The absolute flux measurements in the rocket ultraviolet made by Bohlin, Frimout, and Lillie (BFL) are revised using a more correct treatment of the air extinction that enters the air calibration of their instrument. The absorption by molecular oxygen and ozone, Rayleigh scattering, and extinction by aerosols is tabulated for general use in ultraviolet calibrations performed in air. The revised absolute flux of Eta UMa and final fluxes for Alpha Lyr and Zeta Oph are presented in the 1750 A-3350 A region. The absolute flux of the star Eta UMa (B3 V) is compared to four other independent determinations in the 1200 A-3400 A region and a maximum difference of 35% is found near 1500 A between the OAO-2 and Apollo 17 fluxes. Longward of 1700 A the typical scatter in the different determinations is only plus or minus 5%. The rocket measurements of BFL, the ANS and TD-1 satellite data, and the Apollo 17 data are compared to the ultraviolet fluxes from the OAO-2, demonstrating a photometric reproducibility of about plus or minus 3%. Therefore, all four sets of spectrophotometry can be reduced to a common absolute scale.

Ultraviolet observations of the diffuse galactic light from the S2/68 sky-survey telescope

The diffuse galactic light has been measured in four wavebands centred on 2740, 2350, 1950 and 1550 Å from the background data of the S2/68 sky-survey telescope in the TD-1 satellite. Comparison of the data with the theoretical predictions from a radiative transfer model of the Galaxy has yielded values of the albedo of the interstellar dust.

Observations of interstellar ultraviolet extinction from the S2/68 experiment in the TD-1 satellite

Observations of interstellar ultraviolet extinction from the S2/68 experiment in the TD-1 satellite

An ultraviolet subdwarf companion to HD17576

UTRAVIOLET objects detected by the S2/68 UV sky-survey telescope1 aboard the TD-1 satellite consist primarily of known early-type stars. In addition, there is a substantial subgroup of unidentified and/or untyped ultraviolet stars including some new hot subdwarfs2,3 and a small subgroup of known late-type objects including the 20 stars of types F and G listed in the Ultraviolet Bright-Star Spectrophotometric Catalogue4. Selection of those objects whose ultraviolet colours suggest that they are earlier than an unreddened B5 V star ought to eliminate the latter category entirely, but several apparently late-type anomalies persist. If these objects cannot be discarded on grounds of blending, misidentification, inaccurate ultraviolet colours (objects with low flux or few orbital passes), or erroneous prior spectral types taken from thp literature, then one is confronted with the possibility of genuine duplicity. UV0246–37, taken from the UCL Catalogue of Ultraviolet Objects soon to be published, is just such an object: a binary system consisting of a previously unrecognised hot ultraviolet star and a cooler G-type primary which wholly dominates in the visible region.

Infrared photometry of early-type stars – I

K (2.2 μm) magnitudes are given for reddened early-type stars in the galactic anticentre region |$160^\circ \lt l \lt230^\circ$|⁠, in conjunction with ultraviolet photometry from the TD1 satellite. A value of 3.12 ± 0.05 for the extinction ratio R is derived for the general field and for the Mon OB2 region. The strength of the interstellar absorption feature at 2200 Å is calibrated in terms of AV. Many of the emission-line stars show infrared excess, and there are indications that in some of these the 2200 Å absorption is depleted.

Ultraviolet Photometry from the S2/68 Observations in the TD1 Satellite

SummaryAn ultraviolet photometric system based on observations obtained from the ultraviolet Sky Survey Telescope in the TD1 satellite is described. The system considered here consists of ultraviolet magnitudes at λ1 = 2740A, λ2 = 2190A and λ3 = 1490A. The extinction free parameters derived from the observed ultraviolet colours enable determination of interstellar reddening, spectral type and luminosity. This photometric system has been evaluated by comparing these parameters with other classification parameters.

Spectral Classification of Early Type Stars from the Low Dispersion Ultraviolet Spectra

The methods of spectral classification from the low dispersion ultraviolet spectra obtained with the S2/68 experiment in the TD1 satellite have been described. The bright stars, the spectra of which are photometrically accurate, can be divided into natural groups according to the spectral appearance of the features. These features vary in strength with spectral type and luminosity, and enable separation between main sequence and luminous stars. The limits for these stars areat BO toat AO. For fainter stars the spectral data have been combined to obtain narrow band magnitudes at several wavelengths. These photometric bands have an effective width of 100 A. An ultraviolet photometric system which enables determinations of spectral type and luminosity of early type stars is described and the results for about 3000 stars is presented. The photometric system considered here consists of the ultraviolet colour indices (m2740−V,) (m2190−V) and (m1490−V).

Behaviors of B Star Continua and Absorption Features Determined from the TD1 S2/68 “Ultraviolet Bright Star Spectrophotometric Catalogue” and from Copernicus Spectra

Ultraviolet low resolution spectra of most of the B stars observed with the S2/68 experiment onboard the TD1 satellite in the spectral range 1350 A - 2500 A are computer-investigated in order to define a grid of standard behaviors (J.M. Vreux and J.P. Swings, 1976 Astron. Astrophys., in press). The behavior of the relative intensities of the Balmer and Paschen continua is studied as a function of the spectral type and the visible photometric index Q: both relations are shown to be luminosity dependent. A comparison to theoretical predictions and to previous studies is also presented. A measure of the slope of the pseudo-continuum drawn between ~ λ 1660 A and ~ λ 2550 A is studied in the same way: the effect of luminosity is discussed in connection with recently proposed temperature scales for super-giants. The behavior of the depths of the absorption features at λλ 1550 A, 1940 A, 2000 A, 2055 A, 2105 A, 2340 A and 2395 A with respect to the spectral type, the Q index and the luminosity class is also briefly presented. Special emphasis is given to the absorption features at λ 1550 A in connection to the discussion of the Felll lines in the ultraviolet spectra of early B stars by J.P. Swings, M. Klutz, J.M. Vreux and E. Peytremann (1976, Astron. Astrophys. Suppl. 25, 193). Preliminary results of a study of high resolution Copernicus spectra in the A 1550 A region are also presented: it is shown that the role of Felll remains predominant for stars of spectral type as early as about BIII - BIIII (J.P. Swings and J.M. Vreux, to be published).

On the optical observation of galaxies with large redshifts

IT is well known that the energy distributions of spiral galaxies rise sharply in the ultraviolet. Recent observations of six Sb and Sc galaxies1 made with the TD1 satellite place this steep rise at considerably longer wavelengths than the earlier observations made with the OAO-2 satellite2,3. If the TD1 results are correct, the large ultraviolet flux, which is now considerably closer to the U band, suggests the possibility of detecting more distant galaxies with deep ultraviolet plates than the usual deep B, V or R plates, since this region of high flux will be redshifted into the U band.

Ultraviolet Spectrophotometry of Early-Type Stars

The ultraviolet spectra of B stars obtained with the sky scan telescope in the TD1 satellite have been used to obtain narrow-band magnitudes at several wavelengths. These photometric bands have an effective half-width of 100 Å. We have proposed a two-dimensional classification scheme based on ultraviolet colours, and some preliminary results are presented.