Circumstellar Properties Research Articles

On the Evolutionary Status of Class I Stars and Herbig‐Haro Energy Sources in Taurus‐Auriga

[abridged] We present high resolution optical spectra of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. These spectra are used to search for differences between stars which power Herbig-Haro flows and stars which do not, and to reassess the evolutionary state of so-called protostars (Class I stars) relative to optically revealed T Tauri stars (Class II stars). The stellar mass distribution of Class I stars is similar to that of Class II stars and includes 3 Class I brown dwarfs. Class I stars in Taurus are slowly rotating; the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The mass accretion rates of Class I stars are surprisingly indistinguishable from those of Class II stars; they do not have accretion dominated luminosities. We confirm previous results that find larger forbidden-line emission associated with Class I stars than Class II stars. We suggest that this is caused by an orientation bias that allows a more direct view of the somewhat extended forbidden emission line regions than the obscured stellar photospheres, rather than because of larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of Class I and Class II stars are best explained by a scenario in which most Class I stars are no longer in the main accretion phase and are older than traditionally assumed. Similarly, although stars which power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of stars that do not power these flows.

Very Low Mass Stars and Brown Dwarfs in Taurus‐Auriga

We present high resolution optical spectra obtained with the HIRES spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs (LMTTs) in Taurus-Auriga. Of particular interest is the previously classified continuum T Tauri star GM Tau, which has a spectral type of M6.5 and a mass just below the stellar/substellar boundary. None of the LMTTs in Taurus are rapidly rotating (vsini 270 km/s are classical T Tauri stars (i.e. accreting), independent of stellar spectral type. Although LMTTs can have accretion rates comparable to that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass accretion rate appears to decrease with decreasing mass. The diminished frequency of accretion disks for LMTTs, in conjunction with their lower, on average, mass accretion rates, implies that they are formed with less massive disks than higher-mass T Tauri stars. The radial velocities, circumstellar properties and known binaries do not support the suggestion that many of the lowest mass members of Taurus have been ejected from higher stellar density regions within the cloud. Instead, LMTTs appear to have formed and are evolving in the same way as higher-mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

Very Low Mass Stars and Brown Dwarfs in Taurus

We present high resolution optical spectra obtained with the Keck I telescope of low mass T Tauri stars and brown dwarfs in the Taurus star forming region. Based on the inferred photospheric and circumstellar properties, we conclude that objects in Taurus with masses as low as 50 Jupiters form and evolve in the same way as higher-mass T Tauri stars, but with smaller disks and shorter disk lifetimes.

Observational Constraints on the Formation and Evolution of Binary Stars

We present a high spatial resolution multiwavelength survey of 44 young binary star systems in Taurus-Auriga with separations of 10-1000 AU. These observations, which were obtained using the Hubble Space Telescope and the NASA Infrared Telescope Facility, quadruple the number of close (less than 100 AU) binary stars with spatially resolved measurements from 0.3 to 2.2 μm and are the first 3.6 μm measurements for the majority of the companion stars in the sample. Masses and ages are estimated for the components observed at optical wavelengths. The relative ages of binary star components are more similar than the relative ages of randomly paired single stars within the same star-forming region. This is the first statistically significant evidence for coeval formation. Only one of the companion masses is substellar, from which we conclude that the apparent overabundance of T Tauri star companions relative to main-sequence star companions is not due to a wealth of substellar secondaries that would have been missed in main-sequence surveys. The circumstellar environments of binary star systems are studied in this work through three diagnostics: the infrared color K-L, the ultraviolet excess ΔU, and Hα emission. Several conclusions are drawn. First, the mass accretion rates for primary stars are similar to single stars, which suggests that companions as close as 10 AU have little effect on the mass accretion rate. Second, although most classical T Tauri star binaries retain both a circumprimary and a circumsecondary disk, there are several systems with only a circumprimary disk. Systems with only a circumsecondary disk are rare. This suggests that circumprimary disks survive longer than circumsecondary disks. Third, primary stars accrete at a higher rate, on average, than secondary stars. This is most likely because of their larger stellar mass, since the mass accretion rates for both single and binary T Tauri stars exhibit a moderate mass dependence. Fourth, approximately 10% of T Tauri binary star components have very red near-infrared colors (K-L > 1.4) and unusually high mass accretion rates. This phenomenon does not appear to be restricted to binary systems, however, since a comparable fraction of single T Tauri stars exhibit the same properties. These high accretion rate stars are probably not at an earlier stage of evolution, as has been proposed. Their semblance of younger protostars at optical and infrared wavelengths is most likely because of their similar high levels of accretion, which are above the norm for T Tauri stars, and not because of similar ages. The stellar and circumstellar properties are also used to trace indirectly the evolution of circumbinary material. In contrast to single T Tauri stars, which have disk dissipation timescales comparable to their ages, the disk dissipation timescales for binary T Tauri stars are ~10 times less than their ages. Replenishment of the inner circumstellar disks may be necessary to explain the continuing disk accretion in these systems. The longer disk lifetimes of circumprimary disks, despite their higher depletion rates, suggest that circumprimary disks are being preferentially replenished, possibly from a circumbinary reservoir with low angular momentum relative to the binary. Further support for circumbinary reservoirs comes from the observed correlated presence of circumprimary and circumsecondary disks for binaries with separations of less than ~200 AU. The presence of disks appears uncorrelated for wider binaries. Additionally, binaries with separations of less than ~100 AU exhibit a higher fraction of high mass ratio (ms/mp) pairs than wider binaries. These separation-dependent properties can be explained if the components are being replenished from a common circumbinary reservoir with low angular momentum. The components of the closest pairs are expected to be more equally replenished than the widest pairs, which consequently sustains both disks and drives their mass ratios toward unity. Overall, the results of this study corroborate previous work that suggests that fragmentation is the dominant binary star formation mechanism in Taurus-Auriga; disk instabilities and capture seem unlikely.

Keck Diffraction‐limited Imaging of the Young Quadruple Star System HD 98800

This paper presents diffraction-limited 1-18 μm images of the young quadruple star system HD 98800 obtained with the W. M. Keck 10 m telescopes using speckle and adaptive optics imaging at near-infrared wavelengths and direct imaging at mid-infrared wavelengths. The two components of the visual binary, A and B, both themselves spectroscopic binaries, were separable at all wavelengths, allowing us to determine their stellar and circumstellar properties. Combining these observations with spectroscopic data from the literature, we derive an age of ~107 yr, masses of 0.93 and 0.64 M☉, and an inclination angle of 58° for the spectroscopic components of HD 98800 B and an age of ~107 yr and a mass of 1.1 M☉ for HD 98800 Aa. Our data confirm that the large mid-infrared excess is entirely associated with HD 98800 B. This excess exhibits a blackbody temperature of 150 K and a strong 10 μm silicate emission feature. The theoretical equilibrium radius of large, perfectly absorbing, 150 K grains around HD 98800 B is 2.4 AU, suggesting a circumspectroscopic binary distribution. Our observations set important upper limits on the size of the inner dust radius of ~2 AU (from the mid-infrared data) and on the quantity of scattered light of less than 10% (from the H-band data). For an inner radius of 2 AU, the dust distribution must have a height of at least 1 AU to account for the fractional dust luminosity of ~20% LB. Based on the scattered-light limit, the dust grains responsible for the excess emission must have an albedo of less than 0.33. The presence of the prominent silicate emission feature at 10 μm implies dust grain radii of 2 μm. The total mass of the dust is greater than 0.002 M⊕. We conclude that the dust is located in a circumbinary disk around the HD 98800 B spectroscopic binary with an inner gap of ~2 AU and a height of 1 AU, and we speculate that the A-Bc orbital dynamics are responsible for the characteristics of the observed dust in the system.

Circumstellar Disks in the Orion Nebula Cluster

We combine our previous optical spectroscopic and photometric analysis of ~1600 stars located in the Orion Nebula Cluster (ONC) with our own and published near-infrared photometric surveys of the region in order to investigate the evidence for and properties of circumstellar disks. We use the near-infrared continuum excess as our primary disk diagnostic, although we also study sources with Ca II triplet emission and those designated as proplyds. The measured near-infrared excess is influenced by (1) the presence or absence of a circumstellar disk, (2) the relative importance of disk accretion and inner disk holes, (3) the relative contrast between photospheric and disk emission, and (4) system inclination. After attempting to understand the effects of these influences, we estimate the frequency of circumstellar disks and discuss the evidence for trends in the disk frequency with stellar mass (over the mass range <0.1–50 M_⊙), stellar age (over the age range <0.1–2 Myr), and projected cluster radius (over the radial range 0–3 pc). We find that the fraction of stars retaining their inner (<0.1 AU) circumstellar disks to the present time is at least 55% and probably no more than 90%, averaged over the entire range in stellar mass and stellar age represented in the ONC and over the entire area of our survey. We find no trend in the disk fraction with stellar age, at least not over the limited age range of the cluster. We find that more massive stars are less likely to have disks, consistent with a scenario in which the evolutionary timescales are more rapid for disks surrounding more massive stars than for disks surrounding less massive stars. We also find that the disk frequency begins to decrease toward the lowest masses, although objects of all masses (including those that appear to be substellar) can have disks. We find that the disk frequency increases toward the cluster center. We then argue, using several lines of evidence, that a large fraction of the disks associated with stars in the ONC are accretion disks. The observed trends with stellar age, stellar mass, and projected cluster radius in the disk frequency may, in fact, be driven primarily by trends in the disk accretion properties. From the magnitude of the near-infrared excess above that expected from pure irradiation disks, we find an accretion disk fraction among the stars identified as having disks of 61%–88%. In addition, approximately 20% of the stars in our optical spectroscopic sample show broad (several hundred km s^(-1) FWHM) Ca II emission lines, which are features often associated with accretion disk/wind phenomena; another 50% of the sample have Ca II lines that (at our spectral resolution) are filled in, indicating an independently derived accretion disk frequency of ~70%. Finally, we discuss the near-infrared and optical emission-line properties of that portion of our sample identified from Hubble Space Telescope imaging as having a dark silhouette or an externally ionized structure. This sample, proposed in the literature to have accretion disks, appears to be no different in terms of its stellar or circumstellar properties from the rest of the ONC population. The only feature distinguishing these objects from their ONC siblings thus may be their current (but short-lived) proximity to the massive stars near the cluster center.

Circumstellar shells and mass loss rates: Clues to the evolution of S stars

It is the purpose of this paper to rediscuss the circumstellar properties of S stars and to put these properties in perspective with our current understanding of the evolutionary status of S stars, in particular the intrinsic/extrinsic dichotomy. This dichotomy states that only Tc-rich (intrinsic”) S stars are genuine thermally-pulsing asymptotic giant branch stars, possibly involved in the M–S–C evolutionary sequence. Tc-poor S stars are referred to as extrinsic” S stars, because they are the cooler analogs of barium stars, and like them, owe their chemical peculiarities to mass transfer across their binary system. Accordingly, an extensive data set probing the circumstellar environment of S stars (IRAS flux densities, maser emission, CO rotational lines) has been collected and critically evaluated. This data set combines new observations (9 stars have been observed in the CO line and 3 in the CO line, with four new detections) with existing material (all CO and maser observations of S stars published in the literature). The IRAS flux densities of S stars have been re-evaluated by co-adding the individual scans, in order to better handle the intrinsic variability of these stars in the IRAS bands, and possible contamination by Galactic cirrus. In the () color-color diagram, S stars segregate into five distinct regions according to their Tc content and ZrO/TiO, C/O and IR spectral indices. Stars with photospheric colors (populating Region A”) may be identified with extrinsic S stars. For the other regions characterized by different excess levels in the 12, 25 and 60 μ m bands, several diagnostics (like the IRAS spectral class, maser emission, and shape of CO rotational lines) have been collected to infer the physical properties of the dust shell. A simple radiative-transfer code has also been used to infer the chemical nature (carbonaceous or silicate) of the dust grains from the observed IR colors. S stars with large excesses and moderate [25] - [60] excesses (populating Regions B and C) exhibit the signatures of oxygen-rich shells (9.7 μ m silicate emission and SiO maser emission). The situation is less clear for S stars with small and moderate indices (populating Regions D and E). Their IR colors are consistent with carbonaceous grains (as is their featureless IRAS spectrum, and absence of silicate or SiO maser emission), but these features may equally well be explained by a detached shell. For many of these stars with a large 60 μ m excess, the shell is indeed resolved by the IRAS beam at 60 μ m. The prototypical SC star FU Mon is among these. Since SC stars are believed to be in a very short-lived evolutionary phase where C/O = 1 within 1%, FU Mon may be a good candidate for the interrupted mass-loss” scenario advocated by Willems & de Jong (1988). The CO line profile of FU Mon is also peculiar in being quite narrow ( km s), suggesting that the mass loss has just resumed in this star. Mass loss rates or upper limits have been derived for all S stars observed in the CO rotational lines, and range from y-1 for extrinsic S stars to y-1 (the Mira S star W Aql). These mass-loss rates correlate well with the color index, which probes the dust loss rate, provided that y-1 . Small mass-loss rates are found for extrinsic S stars, consistent with their not being so evolved (RGB or Early-AGB) as the Tc-rich S stars. This result does not support the claim often made in relation with symbiotic stars that binarity strongly enhances the mass-loss rate.

High Spatial Resolution Imaging of Pre–Main‐Sequence Binary Stars: Resolving the Relationship between Disks and Close Companions

In order to study the passive and active effects of young close companion stars and to constrain possible multiple star formation mechanisms, we have carried out a high-resolution imaging program of six multiple T Tauri star systems using the Hubble Space Telescope from 0.3 to 0.9 μm and ground-based speckle imaging from 1 to 2 μm. Each system (GG Tau, UZ Tau, DF Tau, RW Aur, V773 Tau, and V410 Tau) has at least one binary pair with separation between 007 and 04 (10-50 AU). In addition to the previously known companions, a third component is discovered in the V410 Tau system. Many of the companion stars contribute significantly to the unresolved spectral energy distribution and thereby bias the inferred stellar and circumstellar properties. Age is the most systematically biased quantity; neglecting a companion causes an overestimate in age by a factor of ~2 typically and more than 10 in the extreme. Excess emission above the photospheric level is detected at ultraviolet (UV) and near-infrared (NIR) wavelengths for several of the components, which suggests that some close (10-50 AU) T Tauri binaries support at least a minimal circumstellar disk at an age of ~1 Myr. These inner disks not only provide a potential reservoir of material for planetary formation but also may play a key role in the evolution of the components' angular momenta. Possible multiple star formation scenarios are explored in light of the components' relative stellar and circumstellar properties for systems studied here as well as wider and closer systems whose properties are reported in the literature. In summary, the components (1) are coeval, (2) have secondary mass and mass ratio distributions that appear to be independent of both the primary stars' mass and the binary separation, and (3) have UV and NIR excess emission that is either similar or more dominant in the primary. These properties support a scenario in which binary stars form via core fragmentation.

Circumstellar Properties of S Stars. I. Dust Features

view Abstract Citations (53) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Circumstellar Properties of S Stars. I. Dust Features Chen, P. S. ; Kwok, Sun Abstract The 1435 stars currently classified as of S spectral type have been searched for association with the IRAS Point Source Catalog. Among the 835 S stars with IRAS counterparts, 149 are found to have usable IRAS low-resolution spectra. The circumstellar properties of these S stars are classified using the LRS. We find that the circumstellar properties of S stars are very similar to those of M stars. For the S stars that have LRS, silicate emission objects form the largest group. Only four objects of the whole sample show the SiC emission feature commonly found in carbon stars. Some of the S stars that show a photospheric continuum with no infrared excess may not be thermal pulsing AGB stars but are mass transfer binaries. We propose a new classification scheme for AGB stars, taking into account both the photospheric and circumstellar spectral properties. This scheme recognizes the importance of mass loss on the evolution of AGB stars. The evolutionary status of S stars is discussed in terms of their circumstellar properties, and an evolutionary scenario is proposed to reconcile the visible photospheric properties and the circumstellar properties of S stars. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173276 Bibcode: 1993ApJ...416..769C Keywords: ISM: DUST; EXTINCTION; STARS: AGB AND POST-AGB; STARS: CIRCUMSTELLAR MATTER full text sources ADS | data products SIMBAD (149)