David A. Weintraub

Evidence for Accretion: High-Resolution X-Ray Spectroscopy of the Classical T Tauri Star TW Hydrae

We present high-resolution X-ray spectra of the X-ray-bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25 A. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne IX and O VII indicate densities near log ne = 13. A flare with rapid (~1 ks) rise time was detected during our 48 ks observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not seem strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming that accretion powers the X-ray emission, our results for log ne suggest an accretion rate of ~10-8 M☉ yr-1.

Submillimeter measurements of T Tauri and FU Orionis stars

Millimeter and submillimeter detections of 12 low-mass, premain-sequence stars are reported: nine T Tauri stars, one SU Aurigae star, and two FU Orionis stars. The submm spectra of T Tauri stars are found to be less steep than that expected from optically thin thermal dust emission. The dust emissivity index, beta, is found to range between 0 and 1.5 for the T Tauri stars. For the two FU Ori stars in our sample, beta is greater than 1 which is more similar to the beta index range 1-2 found for dust associated with molecular clouds and compact H II regions. The low values for beta indicate that column density and temperature gradients cannot be neglected for the dusty T Tauri environments and/or that the grains are large in the accretion disks surrounding T Tauri stars. Although most of the stars in the sample appear unresolved, the stars T Tau, DG Tau, and possibly HL Tau appear to have structure on 1000-2000 AU scales at 800 microns. Total circumstellar masses of a few times 0.01 solar masses for the T Tauri stars and greater than 0.1 solar masses for the FU Ori stars are estimated. 24 refs.

On the Similarity of FU Orionis Stars to Class I Protostars: Evidence from the Submillimeter

On the basis of analysis of new submillimeter maps of a sample of most known FU Orionis stars, we confirm that these stars are extremely young, more similar to Class I protostars than to Class II T Tauri stars. The submillimeter maps reveal that FU Orionis stars have accretion disks that are larger and contain an order of magnitude more mass than those of T Tauri stars but are comparable in mass to those seen around Class I sources. The disks around the FU Orionis stars are sufficiently massive to act as reservoirs of material for replenishing the mass of the disks after eruptions, in which large amounts of material are believed to fall onto the underlying stars. We have identified a number of FU Orionis stars that are surrounded by extended cloud emission, and in several cases we find nearby submillimeter sources that have no optical or near-infrared counterparts. These sources most likely are young protostellar Class I or Class 0 sources. In at least in one case, V1735 Cyg, the dominant submillimeter source is the protostar, not the optically visible FU Orionis star.

Detections of Rovibrational H2 Emission from the Disks of T Tauri Stars

We report the detection of quiescent H_2 emission in the v=1-0 S(1) line at 2.12183 um in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R 60,000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H_2 emission line lie in the range 0.05-0.10 \AA and, in each case, the central velocity of the emission line is centered at the stars systemic velocity. The line widths range from 9 to 14 km s^{-1}, in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances > 10 AU from the sources. We suggest that UV fluorescence and X-ray heating are likely candidates responsible for producing the observed emission. We present mass estimates from the measured line intensities and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v=1-0 S(1) line suggests that we have detected reservoirs of hot H_2 gas located in the low density, upper atmospheres of circumstellar disks of these stars.

Evidence for Chemical Processing of Precometary Icy Grains In Circumstellar Environments of Pre-Main-Sequence Stars

We report the detection of a broad absorption feature near 2166/cm in the spectrum of the Taurus cloud cource Elias 18. This pre-main-sequence source is the second in Taurus, the third in our survey, and the fifth known in the sky to show the broad 2166/cm absorption feature. Of equal importance, this feature is not seen toward several other embedded sources in our survey, nor is it seen toward the source Elias 16, located behind the Taurus cloud. Laboratory experiments with interstellar ice analogs show that such a feature is associated with a complex C triple bonded to N containing compound (called X(C triple bonded to N)) that results from high-energy processing (ultraviolet irradiation or ion bombardment) of simple ice components into more complex, organic components. We find a nonlinear anticorrelation between the abundance of X(C triple bonded to N) and frozen CO components in nonpolar lattices. We find no correlation between the abundance of X(C triple bonded to N) and frozen CO in polar lattices. Because the abundances of frozen CO and H2O are strongly correlated with each other and with visual extinction toward sources embedded in and located behind the Taurus molecular cloud, these ice components usually are associated with intracloud material. Our results indicate that X(C triple bonded to N) molecules result from chemical processing of dust grains dominated by nonpolar icy mantles in the local environments of pre-main-sequence stars. Such processing of icy grains in the early solar system may be an important source of organic compounds observed in minor solar system bodies. The delivery of these organic compounds to the surface of the primitive Earth through comet impacts may have provided the raw materials for prebiotic chemistry.

A catalog of pre-main-sequence emission-line stars with IRAS source associations

To aid in finding premain-sequence (PMS) emission-line stars that might have dusty circumstellar environments, 361 PMS stars that are associated with 304 separate IRAS sources were identified. These stars include 200 classical T Tauri stars, 25 weak-lined (naked) T Tauri stars, 56 Herbig Ae/Be stars, six FU Orionis stars, and two SU Aurigae stars. All six of the FU Orionis stars surveyed by IRAS were detected. Of the PMS-IRAS Point Source Catalog (PSC) associations, 90 are new and are not noted in the PSC. The other 271 entries include 104 that are correctly identified in the PSC but have not yet appeared in the literature, 56 more that can be found in both the PSC and in the published and unpublished iterature, and 111 that are in the literature but not in the PSC. Spectral slope diagrams constructed from the 12-, 25-, and 60-micron flux densities reveal unique distributions for the different PMS subclasses; these diagrams may help identify the best candidate PMS stars for observations of circumstellar dust. 30 refs.

A SUBMILLIMETER MAPPING SURVEY OF HERBIG AeBe STARS

We have acquired submillimeter observations of 33 fields containing 37 Herbig Ae/Be (HAEBE) stars or potential HAEBE stars, including SCUBA maps of all but two of these stars. Nine target stars show extended dust emission. The other 18 are unresolved, suggesting that the dust envelopes or disks around these stars are less than a few arcseconds in angular size. In several cases, we find that the strongest submillimeter emission originates from younger, heavily embedded sources rather than from the HAEBE star, which means that previous models must be viewed with caution. These new data, in combination with far-infrared flux measurements available in the literature, yield spectral energy distributions (SEDs) from far-infrared to millimeter wavelengths for all the observed objects. Isothermal fits to these SEDs demonstrate excellent fits, in most cases, to the flux densities longward of 100 μm. We find that a smaller proportion of B-type stars than A- and F-type stars are surrounded by circumstellar disks, suggesting that disks around B stars dissipate on shorter timescales than those around later spectral types. Our models also reveal that the mass of the circumstellar material and the value of β are correlated, with low masses corresponding to low values of β. Since low values of β imply large grain sizes, our results suggest that a large fraction of the mass in low-β sources is locked up in very large grains. Several of the isolated HAEBE stars have disks with very flat submillimeter SEDs. These disks may be on the verge of forming planetary systems.

DETECTION OF QUIESCENT MOLECULAR HYDROGEN GAS IN THE CIRCUMSTELLAR DISK OF A CLASSICAL T TAURI STAR

We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1¨0 S(1) line at 2.1218 km through high-resolution spectros- copy using CSHELL on NASAs Infrared Telescope Facility. The brightness of the observed emission line is consistent with that predicted from models for X-ray excitation of the 1¨0 S(1) line of This H 2 . result demonstrates that X-ray ionization is a plausible mechanism for excitation of the in the H 2 gaseous disks of T Tauri stars. In addition, from these high-resolution spectra we have measured the radial velocities of three T Tauri stars in the TW Hya association, CD (33i7795, HD 98800, and TW Hya. We —nd that the radial velocities of these three stars are very similar to each other and to those of at least three other stars presumed to be members of the TW Hya association, Hen 600A, Hen 600B, and CD (29i8887. This result, combined with the similar proper motions of all six of these stars, lends support to the hypothesis that all of these stars share a common origin in a now-dispersed molecular cloud. Subject headings: infrared: starsopen clusters and associations: individual (TW Hydrae, HD 98800, CD (33i7795) ¨ stars: premain-sequenceX-rays: stars

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor ∼50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeils nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star–disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star.

Submillimeter Maps of Bok Globule Cores: Evidence for Multiple Epoch Star Formation

We mapped 15 cold IRAS sources associated with Bok globules, using the submillimeter common user bolometer array SCUBA on the James Clerk Maxwell Telescope, in order to search for deeply embed- ded protostars. Submillimeter emission was detected in six of these globules, —ve of which contain one or more compact sources. We detected a total of seven compact submillimeter sources. At least —ve of these, which have no known near-infrared counterparts, are candidate or con—rmed class 0 sources. The IRAS sources detected in the submillimeter have IRAS colors consistent with those of dense molecular cloud cores and/or out—ow sources, whereas the IRAS sources not detected in this survey have IRAS colors consistent with those of infrared cirrus. We present compelling evidence that multiple star formation has occurred in four of the six Bok globules that we have detected in the submillimeter; thus, the formation of a single star in isolation from other stars appears to be a rare event in Bok globules. These four globules contain two or three young stellar objects, sometimes at very diUerent evolutionary stages. Two Bok globules, CB 68 and CB 232, are found to contain a candidate class 0 source in addition to a more evolved class I or class II premain-sequence star. These observations, in which we —nd a class 0 proto- star and a premain-sequence star in the same globule, suggest that multiple epochs of star formation are possible in Bok globules. Subject headings: ISM: globulesISM: individual (B335) ¨ stars: formation ¨ stars: premain-sequencesubmillimetersurveys