The depth of the convective envelope on the lower main sequence and the depletion of lithium.
Mechanism of lithium depletion in main sequence stars of solar type with involvement of substantial amount of convective overshoot
MAIN SEQUENCE STAR↗
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The interstellar redding law in the ultraviolet deduced from filter photometry obtained by the OAO-2 satellite
Filter photometry has been obtained of 16 BO stars at ten effective wavelengths in the range 4250-1430 A. The wavelength dependence of the interstellar reddening law, deduced from a least squares fit of the observed values to the reddening line at each band, is found in satisfactory agreement with that derived by Bless and Savage (1972). Toward the shorter wavelengths the increase of the computed probable error of the slope of the mean reddening line suggests that large fluctuations in the law may occur from star to star. Similar computations, separating main-sequence stars and supergiants, indicate that the large fluctuations of the law appear to be well related to the luminosity of the stars; the supergiants show systematically less extinction, this deficiency becoming large toward the far UV. The small number in the sample however, does not allow a general conclusion to be drawn.
Laget, M.↗
Photometric study of NGC 2023 in the 3500 A to 10000 A region - Confirmation of a near-IR emission process in reflection nebulae
A surface brightness study of the reflection nebula NGC 2023 covering the 3500-10,000 A wavelength region performed with uvby photoelectric photometry and BVRI imaging with a CCD detector is reported along with VRI photometry of a cluster of embedded red stars. The nebular radiation in the 3500-5500 A region is dust-scattered starlight originating in the star HD 37903. The embedded red stars are probably pre-main sequence stars. The nebular surface brightness in R and I exceeds that expected on the basis of a reasonable radiative transfer model by factors of two and more than three, respectively. The excess radiation is extended across the nebula in a manner similar to the scattered light. The extended red emission may be interpreted as the high-frequency extension of extended emission discovered by Sellgren, Werner, and Dinerstein (1983) in the 2-5 micron region in NGC 2023 and two other reflection nebulae.
Schild, R. E.↗
Characterizing Rocky and Gaseous Exoplanets with 2 m Class Space-based Coronagraphs
Several concepts now exist for small, space-based missions to directly characterize exoplanets in reflected light. While studies have been performed that investigate the potential detection yields of such missions, little work has been done to understand how instrumental and astrophysical parameters will affect the ability of these missions to obtain spectra that are useful for characterizing their planetary targets. Here, we develop an instrument noise model suitable for studying the spectral characterization potential of a coronagraph-equipped, space-based telescope. We adopt a baseline set of telescope and instrument parameters appropriate for near-future planned missions like WFIRST-AFTA, including a 2 m diameter primary aperture, an operational wavelength range of 0.4–1.0 μm, and an instrument spectral resolution of λ/Δλ =70, and apply our baseline model to a variety of spectral models of different planet types, including Earth twins, Jupiter twins, and warm and cool Jupiters and Neptunes. With our exoplanet spectral models, we explore wavelength-dependent planet–star flux ratios for main-sequence stars of various effective temperatures and discuss how coronagraph inner and outer working angle constraints will influence the potential to study different types of planets. For planets most favorable to spectroscopic characterization—cool Jupiters and Neptunes as well as nearby super-Earths—we study the integration times required to achieve moderate signal-to-noise ratio spectra. We also explore the sensitivity of the integration times required to either detect the bottom or presence of key absorption bands (for methane, water vapor, and molecular oxygen) to coronagraph raw contrast performance, exozodiacal light levels, and the distance to the planetary system. Decreasing detector quantum efficiency at longer visible wavelengths makes the detection of water vapor in the atmospheres of Earth-like planets extremely challenging, and also hinders detections of the 0.89 μm methane band. Additionally, most modeled observations have noise dominated by dark currents, indicating that improving CCD performance could substantially drive down requisite integration times. Finally, we briefly discuss the extension of our models to a more distant future Large UV-Optical-InfraRed (LUVOIR) mission.
Astrobiology↗
Chandra X-ray Observations of Young Clusters: Orion Flanking Fields Data - Volume II
We present results of Chandra observations of two flanking fields (FFs) in Orion, outside the Orion Nebula Cluster (ONC). The observations were taken with the ACIS-I camera with an exposure time of about 48 ks each field. We present a catalog of 417 sources, which includes X-ray luminosity, optical and infrared photometry, and X-ray variability information. We have found 91 variable sources, 33 of which have a flarelike light curve and 11 of which have a pattern of a steady increase or decrease over a 10 hr period. The optical and infrared photometry for the stars identified as X-ray sources are consistent with most of these objects being pre-main-sequence stars with ages younger than 10 Myr. We present evidence for an age difference among the X-ray-selected samples of NGC 2264, Orion FFs, and ONC, with NGC 2264 being the oldest and ONC being the youngest.
pre-main sequence↗
Stellar population of NGC 1850 in the LMC
Results are presented of a study of the stellar population of the young globular cluster NGC 1850 in the LMC based on observations made on the HST. The observed magnitude range corresponds to main-sequence stars with masses in the interval 0.8-13 solar masses. The main sequence has a finite width, definitely larger than the spread produced by the measurement uncertainties, suggesting either a spread in ages (20 percent) or the effect of binary systems with a flat distribution of mass ratios.
Gilmozzi, Roberto↗
The evolution of solar ultraviolet luminosity
Astronomical observations of stars analogous to the sun are used to construct a tentative account of the evolution of solar UV luminosity. Evidence exists that the young sun was a much more powerful source of energetic particles and radiation than it is today, and while on the main sequence, solar activity has declined as an inverse power law of age as a consequence of angular momentum loss to the solar wind. Observations of pre-main sequence stars indicate that before the sun reached the main sequence, it may have emitted as much as ten thousand times the amount of ultraviolet radiation that it does today. The impact of the results on knowledge of photochemistry and escape of constituents of primordial planetary atmospheres is discussed.
Zahnle, K. J.↗
Theoretical studies of chromospheres and winds in cool stars
The atmospheres of hot pre-main sequence stars are discussed along with FU Orionis winds, h alpha emission from M dwarfs, and envelopes of T Tauri stars. In addition, extended chromospheres of M supergiants and metal deficiency giants are considered.
Hartmann, L.↗
Examples of Comet-like Spectra among Beta Pic-like Stars
Beta Pic is a nearby main-sequence star with an extended dust disk, large infrared excess, and mid-infrared emission similar to that seen in some cometary spectra. We selected a sample of stars whose infrared properties resemble Beta Pic and obtained 8 to 13 micron spectra (resolution 200). We find a variety of silicate emission features among the stars in our sample, ranging from classic interstellar dust features to spectra similar to that of comet P/Halley and the laboratory spectra of Interplanetary Dust Particles (IDP's).
Butner, H. M.↗
X-ray sources in regions of star formation. 5: The low mass stars of the Upper Scorpius association
We report followup investigations of Einstein x-ray observations of the Upper Scorpius OB association. We identify 28 low mass pre-main sequence stars as counterparts of x-ray sources in the approximately = 7 square degrees of the OB association observed. Based on kinematics and lithium abundances, these stars are low mass members of the OB association. We use optical spectra and optical and near-IR photometry to determine the stellar luminosities, effective temperatures, masses, and ages. We show that the bolometric corrections and effective temperatures of the G and K stars are consistent with those of subgiants. The low mass stars have isochronal ages of 1-2 Myr, depending on the choice of evolutionary models, with very small dispersion (sigma approximately = 1 Myr). This age is significantly younger than the 5-6 Myr found for the more massive B stars. The small dispersion in stellar ages, less than 10% the sound-crossing time of the association, suggests that star formation was triggered. We present two scenarios for star formation in this association. In the two-episode scenario, formation of the low mass stars was triggered by a supernova explosion, and the low mass stars form quickly, with high efficiency. Alternatively, high and low mass star formation was all initiated at the same time, some 5-6 Myr ago, and the apparent systematic age difference is an artifact of how the isochrones are dated. The effect of the supernova is to terminate mass accretion and yield an apparently coeval population. We show that the incompleteness in the x-ray sampling is about 65%, and is strongly dependent on stellar mass. After correction for incompleteness, we estimate there are about 2000 low mass members (stellar mass less than 2 solar mass) of this association. The mass function in this association is indistinguishable from that of the field. The ratio of naked to classical T Tauri stars is much larger than in Tau-Aur, and may be attributable to the local environment. We also present observations of eight ROX sources associated with the rho Oph cloud, and observations of non-pre-main sequence (PMS) stars in our fields.
Walter, Frederick M.↗
Nature of the secondary component of beta Lyrae.
Development of a hypothesis concerning the nature of the secondary component of beta Lyrae which is regarded as more plausible than the suggestion that this component is a black hole. Adopting Huang's (1962) and Woolf's (1965) basic model for beta Lyrae, it is suggested that the secondary star may be a massive 'main-sequence' star in rapid, nonuniform motion. Certain findings regarding mu Sco and V356 Sgr, which have primaries that are very cool and underluminous for their masses, are cited as evidence for such an inference, since their underluminosity is probably due to high angular momentum and they are normal enough not to be suspected of harboring a black hole.
Stothers, R.↗
Investigation of ultraviolet fluxes of normal and peculiar stars
Data from Project Celescope, a program that photographed the ultraviolet sky, in order to study several problems in current astrophysics are analyzed. Two star clusters, the Pleiades and the Hyades, reveal differences between the two that we are unable to explain simply from their differences in chemical abundance, rotation, or reddening. Data for Orion show large scatter, which appears to be in the sense that the Orion stars are too faint for their ground-based photometry. Similarly, many supergiants in the association Sco OB1 are too faint in the ultraviolet, but the ultraviolet brightness appears to be only poorly correlated with spectral type. Ultraviolet Celescope data for several groups of peculiar stars have also been analyzed. The strong He I stars are too faint in the ultraviolet, possibly owing to enhancement of O II continuous opacity due to oxygen overabundance. The Be stars appear to have ultraviolet colors normal for their MK spectral types. The P Cygni stars are considerably fainter than main-sequence stars of comparable spectral type, probably owing, at least in part, to line blocking by resonance lines of multiply ionized light metals. The Wolf-Rayet stars have ultraviolet color temperatures of O stars.
Deutschman, W. A.↗
Spectral types of stars in the SMC associations NGC456, NGC460a, b and NGC465
Spectral types of stars for an extended area (1200 sq arcmin) of the SMC, containing the associations NGC456, NGC460 a, b and NGC465 have been derived here. High-quality UJ and YJ film copies of objective prism plates taken with the 1.2-m UK Schmidt telescope were used. The medium dispersion spectral (830 A/mm) were digitized with a PDS microdensitometer and spectra types of stars were obtained down to about 16.5 mag. A computer interactive method of reduction has been developed, and the spectral types obtained are accurate to 0.3 spectral subtypes. Spectral classification for the low (2440 A/mm) dispersion spectra down to 18.5 mag was carried out visually using a binocular microscope. The frequency distributions of stars in the associations region, after subtracting the field stars, have revealed that only blue main sequence stars are their members.
Dapergolas, A.↗
An Infrared Image of the Dust Disc Around Beta Pic
The star, B Pictoris, is a normal main sequence star with a remnant disc of cool dust around it. We present observations of the B pic dust disc in the far infrared, obtained using the photometer (ISOPHOT) on the Infrared Space Observatory (ISO) satellite.
photometer↗
Early and Main Sequence Evolution of Stars in the Range 0.5 to 100 Solar Masses
Early and main sequence evolution of stars in 0.5 to 100 solar mass range
Cameron, A. G. W.↗
A possible brown dwarf companion to Gliese 569
A faint cool companion to Gliese 569, discovered during an IR imaging survey of nearby stars, may be the lowest-mass stellar object yet found. The companion is somewhat cooler in its 1.65-3.75-micron energy distribution than the coolest known main-sequence stars, indicating a low mass. Despite its lower temperature, it is more luminous than similar extremely low-mass stars, suggesting that it is either a young low-mass star evolving toward the main sequence or a cooling substellar brown dwarf. The primary star has emission lines and a low space velocity and exhibits flaring, all of which imply youth for this system. Observations of Gliese 569 and its companion over a period of 2 yr confirm the common proper motion expected of a true binary. The 5-arcsec apparent separation (50 AU) implies an orbital period of roughly 500 yr, which will permit an eventual direct determination of the mass of the companion.
Forrest, W. J.↗
Cometary Dust in the Debris Disks of HD 31648 and HD 163296: Two "Baby" (BETA) Pictoris Stars
The debris disks surrounding the pre-main-sequence stars HD 31648 and HD 163296 were observed spectroscopically between 3 and 14 microns. Both stars possess a silicate emission feature at 10 microns that resembles that of the star beta Pictoris and those observed in solar system comets. The structure of the band is consistent with a mixture of olivine and pyroxene material, plus an underlying continuum of unspecified origin. The similarity in both size and structure of the silicate band suggests that the material in these systems had a processing history similar to that in our own solar system prior to the time that the grains were incorporated into comets.
Sitko, Michael L.↗
Cometary Dust in the Debris of HD 31648 and HD163296: Two "Baby" Beta pictoris Stars
The debris disks surrounding the pre-main-sequence stars HD 31648 and HD 163296 were observed spectroscopically between 3 and 14 microns. Both stars possess a silicate emission feature at 10 Am that resembles that of the star P Pictoris and those observed in solar system comets. The structure of the band is consistent with a mixture of olivine and pyroxene material, plus an underlying continuum of unspecified origin. The similarity in both size and structure of the silicate band suggests that the material in these systems had a processing history similar to that in our own solar system prior to the time that the grains were incorporated into comets.
Sitko, Michael L.↗