near-IR Excess Research Articles

Spectroscopy of B-type emission-line stars with compact dusty envelopes: HD 85567, Hen 3-140, and Hen 3-1398

We report the results of our spectroscopic observations (3600{5900 A, R' 16 000 and 5750{8750 A, R' 11 000) of three southern emission-line B{type stars associated with IRAS sources, HD 85567, Hen 3{140, and Hen 3{1398, whose nature and evolutionary state are not well-established yet. We also obtained new multicolour photometry of HD 85567 (UBV(RI)cJHKL) and Hen 3{140 (JHKLM). A large near-IR excess is found in Hen 3{ 140 for the rst time. All three objects show double-peaked Fe ii lines in emission. The Balmer emission lines are double-peaked in HD 85567 and Hen 3{140, while the He i lines are found in absorption. Hen 3{1398 displays triple-peaked Balmer lines as well as single-peaked He i and He ii lines in emission. Fundamental parameters of the stars are estimated on the basis of both photometric and spectroscopic data. The whole set of their properties resemble that of a recently dened group of Be stars with warm dust (Sheikina et al. 2000).

A 3 μ[CLC]m[/CLC] Survey of the Chamaeleon I Dark Cloud

We describe an L-band photometric survey of ~ 0.5 deg2 of the Chamaeleon I dark cloud. The survey has a completeness limit of L < 11.0. Our survey detects 124 sources, including all known pre–main-sequence stars with L ≤ 11. The fraction of sources with near-IR excess emission is 58% ± 4% for K = 9–11. Cha I sources have bluer H-K and K-L colors than pre–main-sequence stars in Taurus-Auriga. These sources also have a strong correlation between EW(Hα) and K-L. Stars with K-L≤ 0.6 have weak Hα emission; stars with K-L≥ 0.6 have strong Hα emission. Because many Cha I sources are heavily reddened, this division between weak emission T Tauri stars and classical T Tauri stars occurs at a redder K-L than in Taurus-Auriga.

A 10 Micron Search for Truncated Disks Among Pre–Main-Sequence Stars with Photometric Rotation Periods

We use mid-IR (primarily 10 μm) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre–main-sequence stars in Orion and in Taurus-Auriga for which rotation periods are known and for which there is no evidence of inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence of excess mid-IR emission. Although these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk locking, and there exist very young stars already rotating at nearly breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr; doing so may allow the models to explain the rotational evolution of low-mass pre–main-sequence stars in a way that does not depend on braking by disks.

EXPORT: Near-IR observations of Vega-type and pre-main sequence stars

We present near-IR J H K photometric data of sample of 58 main-sequence, mainly Vega-type, and pre-main sequence stars. The data were taken during four observation runs in the period May 1998 to January 1999 and form part of a coordinated effort with simultaneous optical spectroscopy and photo-polarimetry. The near-IR colors of the MS stars correspond in most cases tp photospheric colors, although noticealbe reddening is present towards a few objects, and these stars show no brightness variability within the observational errors. On the other hand, the PMS stars show near-IR excesses and variability consistent with previous data.

Spectroscopy and photometry of the emission–line B–type stars AS 78 and MWC 657

We present the results of low- and high- resolution spectroscopic and multicolour photometric observations of two early-type emission-line stars, AS 78 and MWC 657. They were identied by Dong & Hu (1991) with the IRAS sources 03549+5602 and 22407+6008, respectively, among many other sources displaying a very strong infrared excess, V (25) 8 mag AS 78 is recognized as a photometric variable for the rst time. A large near-IR excess radiation is detected in AS 78 and conrmed by new, higher-quality, data for MWC 657. Signicant variations in the Balmer line proles are detected for both objects. Modelling of the Balmer line proles of AS 78 obtained in 1994 yields the following parameters of the star and its wind: Te 17 000 K, log Lbol/L 4.0, _ M 10 6 M yr 1 .T he photometric and new spectroscopic data indicate that the star's Te increases with time, while the stellar wind changes its structure. Distances towards both objects are estimated on the basis of their radial velocities and the galactic rotation curve. Consideration of dierent options about the nature and evolutionary state of both objects leads us to suggest that they are binary systems

Binary “Post-AGB” Stars

In this contribution we report on our radial-velocity monitoring of optically bright, high-latitude supergiants that appear to be in a post-AGB evolutionary stage. Binarity is a widespread phenomenon among our sample stars. More precisely: all objects with a near-IR excess in their energy distribution turn out to be binaries while the fraction of binaries in our program stars with only a far-IR excess is very small. The orbital periods, the often non-zero eccentricities, and the sometimes large mass functions set strong constraints on the previous evolution in which mass transfer must have been an important ingredient. We have accumulated observational evidence that the presence of a circum-binary dusty disk has an important dynamical and sometimes even chemical influence on the binary and its evolution. Some objects with a high mass function still defy an explanation.

Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections

ABSTRA C T We present the results of high-resolution spectroscopic, low-resolution spectrophotometric and spectropolarimetric and broad-band multicolour observations of four B-type stars (HD 4881, 5839, 224648 and 179218) and two A-type stars (HD 32509 and 184761) with strong far-infrared (IR) excesses. The excess in HD 184761, which is located at a distance of 65 pc from the Sun, was recognized for the first time. Double-peaked Ha emission line profiles are found in HD 4881 and HD 5839, while HD 184761, HD 224648 and HD 32509 display no emission in Ha. The remarkable variations observed in the Ha profile of HD 179218 are also observed in some classical Be and Herbig Ae/Be stars. An intrinsic component of polarization is clearly present in HD 179218, only an interstellar component is detected in HD 4881 and HD 224648, and HD 184761 was found to be unpolarized. Improved effective temperatures for all six objects were derived. Parallaxes measured by the Hipparcos satellite were used to determine positions of the stars in the HR diagram. HD 4881 and HD 5839 are an order of magnitude more luminous than main-sequence stars of similar temperatures and are most likely newly discovered classical Be stars. Study of the high-resolution IRAS maps and modelling of the spectral energy distributions of HD 4881, HD 5839 and HD 224648 suggest that the observed large IR excesses are caused by radiation from circumstellar dust rather than free‐free radiation or infrared cirrus, so they may be higher mass counterparts of b Pictoris stars. HD 32509, HD 224648 and HD 184761, which have very small near-IR excesses, are probably young main-sequence stars. HD 179218, which exhibits the largest near- and far-IR excess in the sample, is an isolated pre-main-sequence Herbig Be star.

The Near-Infrared Extinction Law and Limits on the Pre–Main-Sequence Population of the ρ Ophiuchi Dark Cloud

We describe new techniques to measure the near-infrared extinction law and to place limits on the pre–main-sequence stellar population of a dark cloud. We analyze JHK imaging data for the central 1 deg2 of the ρ Ophiuchi cloud core and show that nearly all stars projected onto regions of low CS intensity, ICS ≤ 10 K km s-1, are background stars. Most sources at larger CS intensities lie within cloud material. We use the background stars to derive the slope of the near-IR extinction law, EJ - H/EH - K = 1.57 ± 0.03. This result is consistent with previous extinction laws but has a factor of 2–3 smaller uncertainty. The new ρ Oph extinction law yields strong constraints on the number of previously undiscovered pre–main-sequence stars in the cloud, 46 ± 11, and the number of previously undiscovered young stars with near-IR excesses, 15 ± 4. Neither limit exceeds the number of known pre–main-sequence stars in the cloud, ~100. Thus, current samples of pre–main-sequence stars are reasonably complete for K ≤ 14.

IRAS 20050+2720: An Embedded Young Cluster Associated with a Multipolar Outflow

We obtained near-IR images at 1-5 μm of IRAS 20050+2720, a very red far-infrared source with a multipolar outflow (as noted by Bachiller et al.). Of the 212 sources we detected, half of them are probably members of an embedded cluster around the IRAS source. More than 50% of the cluster members show near-IR excesses, suggesting that they are pre-main-sequence stars with circumstellar emission. The luminosity functions of the cluster show turnovers and slopes consistent with pre-main-sequence model predictions for an average cluster age of ~1 Myr. Three subclusters are identified within the cluster. One of them (subcluster A) appears to be associated with denser molecular gas and higher extinction than the other two subclusters. It also coincides with the IRAS source, a compact millimeter continuum source, and the center of the multipolar CO outflow. We identified in subcluster A several deeply embedded sources with sharply rising spectral energy distributions. These results support strongly the suggestion that the multipolar outflow is the result of simultaneous star formation within 0.1 pc of the IRAS source. We suggest that the cluster was formed in several episodes of star formation over the last ~1 Myr and that subcluster A represents the most recent episode that formed multiple young stars within ~0.1 Myr.

The link between barium stars and optically bright post-AGB binaries

Systematic radial velocity monitoring programs revealed that all optically bright, high-latitude post-AGB stars that show a near-IR excess in their energy distribution are binaries (see the contribution of R. Waters et al. in these proceedings). A standard AGB evolution for the shorter period objects seems, however, impossible since the orbits are too small to accommodate an AGB star! Non-standard phenomena connected to the specific binary nature have to be invoked in order to understand these systems.

The β Pictoris Phenomenon in A-Shell Stars: Detection of Accreting Gas

We present the results of an expanded survey of A-shell stars using IUE high-dispersion spectra and find accreting, circumstellar gas in the line of sight to nine stars, in addition to the previously identified beta Pic, HR 10, and 131 Tau, which can be followed to between +70 and 100 km/s relative to the star. Two of the program stars, HD 88195 and HD 148283, show variable high-velocity gas. Given the small number of IUE spectra for our program stars, detection of high-velocity, accreting gas in 2/3 of the A-shell stars sampled indicates that accretion is an intrinsic part of the A-shell phenomenon and that beta Pic is not unique among main-sequence A stars in exhibiting such activity. Our program stars, as a group, have smaller column densities of high-velocity gas and smaller near-IR excesses compared with beta Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with beta Pic, and suggest that the majority of field A-shell stars are older than beta Pic.

The spectral energy distribution of the z=3 quasar: HS 1946+7658

For the bright z = 3.02 radio-quiet quasar, HS 1946+7658, we have obtained radio to X-ray data within the past year: 5 GHz and 1415 MHz data from the Very Large Array (VLA); IR photometry at J, H, K, L prime (3.4 micrometers and N; IR spectroscopy; UBVRI photometry; optical spectrophotometry and high-resolution spectra; and an X-ray spectrum from the ROSAT Position Sensitive Proportional Counter (PSPC). The spectral energy distribution (SED) constructed from these data is compared to the mean SED for a set of low-redshift quasars, and while they appear generally similar, there are several differences. In relation to the low-redshift mean, the SED of HS 1946+7658 shows (1) only an upper limit at 10 micrometers (a rest wavelength of 2.5 micrometers, indicating that HS 1946+7658 does not have a strong near-IR excess such as hot dust would produce; (2) relatively weak Fe II and Balmer continuum emission at approximately 3000 A; and (3) a steeper turndown shortward of Ly alpha, even after correction for the Ly alpha forest absorption lines.

The atmospheres of T Tauri stars. II - Chromospheric line fluxes and veiling

view Abstract Citations (30) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Atmospheres of T Tauri Stars. II. Chromospheric Line Fluxes and Veiling Batalha, C. C. ; Basri, Gibor Abstract A set of high-resolution calibrated spectra of southern T Tauri Stars (TTS), comprising objects with H-alpha equivalent widths ranging from 3 to 40 A are used to derive one-component plane-parallel chromospheric models. Our motivation is to discover how much emission activity can truly be ascribed to stellar activity, using real calibrated stellar data. We find that simple models can reproduce the Ca II line at 8542 A for most of the stars. The chromospheric enhancements are comparable to the ones found among active main-sequence stars of similar spectral type. These models cannot account for the veiling observed in TTS. Likewise, realistic TTS chromospheres are unable to create the observed Balmer jumps. The targets have a range of H-alpha strength that allow us to study possible correlations among narrow symmetric emission lines, and H-alpha which probes nonstellar regions. On the basis of their stellar vs systemic luminosities, Sz 77, Sz 82, and Sz 19 present signatures of disk accretion, in addition to the previously reported case of Sz 98. By correlating the fluxes of the Ca II IR line, H-alpha and the near-IR excess, we conclude that disk accretion may enhance the luminosity of typical chromospheric emission lines. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172926 Bibcode: 1993ApJ...412..363B Keywords: Chromosphere; Stellar Atmospheres; Stellar Spectra; T Tauri Stars; Emission Spectra; H Alpha Line; Spectral Line Width; Stellar Activity; Stellar Luminosity; Stellar Mass Accretion; Stellar Models; Astrophysics; STARS: PRE--MAIN-SEQUENCE; STARS: CHROMOSPHERES; LINE: FORMATION; RADIATIVE TRANSFER full text sources ADS | data products SIMBAD (6) Related Materials (1) Part 1: 1987ApJ...318..823F

X-ray irradiation of interstellar grains in active galaxies - Evaporation and infrared spectra

view Abstract Citations (79) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray Irradiation of Interstellar Grains in Active Galaxies: Evaporation and Infrared Spectra Voit, G. M. Abstract X-ray irradiated dust grains are likely to be present in Seyfert galaxies, quasars, starburst galaxies, and perhaps near X-ray sources in our own Galaxy. Infrared observations have shown that small grains (<10 A) subjected to ambient interstellar UV radiation will heat sporadically to more than 1000 K and emit in the near-infrared. X-rays will heat even larger grains in this way. A population of X-ray- illuminated grains varying in size will radiate an infrared spectrum with a long-wavelength blackbody peak, at the equilibrium temperature of the large grains, plus some excess nonequilibrium continuum emission extending into the 1-3 micron range. X-rays will efficiently evaporate dust grains smaller than 10 A, so X-ray illuminated dust should have no polycyclic aromatic hydrocarbon (PAH) features. The turnover shortward of 3 microns is robust and represents the onset of evaporative cooling in grains. The characteristics of the IR spectrum from X-ray-heated grains are not particularly sensitive to the shape of the incident X-ray spectrum; however, UV emission stimulated by X-ray photoelectrons originating in the gas phase can substantially increase the strength of the near-IR excess. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170490 Bibcode: 1991ApJ...379..122V Keywords: Active Galaxies; Cosmic Dust; Evaporation; Interstellar Matter; Quasars; Seyfert Galaxies; X Ray Irradiation; Infrared Spectra; Ionization Cross Sections; Numerical Integration; Optical Thickness; Photoionization; X Ray Spectra; Astrophysics; GALAXIES: INTERSTELLAR MATTER; GALAXIES: SEYFERT; GALAXIES: X-RAYS; INFRARED: SPECTRA; INTERSTELLAR: GRAINS; QUASARS full text sources ADS | data products SIMBAD (1)

Circumstellar material associated with solar-type pre-main-sequence stars - A possible constraint on the timescale for planet building

The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time.

Atmospheres of T Tauri stars - The photosphere and low chromosphere

view Abstract Citations (81) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Atmospheres of T Tau stars : the photosphere and low chromosphere. Cram, L. E. Abstract A model for the photospheric and low chromospheric regions of the atmosphere of an advanced T Tauri star has been constructed, by superposing an ad hoc temperature rise on a theoretical model photosphere. This model has been used to synthesize several spectral features, including the continuum from 300 A to 300 microns and the Fe I and Fe II spectrum in the interval 4475-4500 A. A comparison between these predictions and observations supports the conclusion that T Tauri stars possess a chromosphere that begins deep in the atmosphere, with the beginning depth of the chromosphere principally determining the degree of advancement of the T Tauri spectrum. There are a number of spectral features that cannot be explained by this model, including the near-IR excess, the large H alpha:H beta decrement, and the asymmetry of emission lines. Publication: The Astrophysical Journal Pub Date: December 1979 DOI: 10.1086/157578 Bibcode: 1979ApJ...234..949C Keywords: Chromosphere; Photosphere; Stellar Models; Stellar Temperature; T Tauri Stars; Emission Spectra; Line Spectra; Spectrum Analysis; Stellar Atmospheres; Astrophysics full text sources ADS | data products SIMBAD (3)