Circumstellar Gas Disk Research Articles

Evidence for weak magnetic fields in early‐type emission stars

AbstractWe report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi‐mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

“The photopolarimetric activity and circumstellar environment of the T tauri star CO ORI”

We present the results of simultaneous UBVRI photometry and polarimetry of the classical T Tauri star CO Ori carried out at the Crimean Astrophysical Observatory during the 18 years between 1986 and 2004. We show that the variations of linear polarization accompanying the star’s brightness variations follow the law characteristic of UX Ori stars. This suggests that the brightness variations of the star are mainly due to changes of the circumstellar extinction due to non-uniform structure of the circumstellar environment, and to an “optimal” orientation of the circumstellar gas and dust disk relative to the observer, whose line of sight crosses the gas and dust atmosphere of the disk. We determine the star’s intrinsic polarization due to scattering of light in the circumstellar disk. The polarization position angle indicates the orientation of the disk’s symmetry axis in the plane of the sky. Our analysis of an archival light curve for CO Ori confirms the existence of a many-year cycle of photometric activity, suspected by us earlier. The refined period of this cycle is 12.4 years. The existence of such activity cycles of UX Ori stars testifies to considerable deviations of their circumstellar disks from axial symmetry, a reflection of either stellar binarity or the commencement of the process of planetary formation.

First Ultraviolet Spectrum of a Brown Dwarf: Evidence for H 2 Fluorescence and Accretion

We analyze an HST STIS ultraviolet spectrum of the young brown dwarf 2MASSW J1207334-393254, a member of the 10 million year old TW Hya association that has a planetary-mass companion. We detect and identify numerous emission lines. C IV and other ions are seen that arise in hot gas. We identify a series of lines with Lyman-pumped H2 molecular lines, indicating that cool gas is also present. Overall, this substellar object shows many of the same characteristics as classical T Tauri stars. We interpret our results as direct evidence of accretion from a circumstellar gas disk, consistent with previous claims. The lack of Si IV emission from the accreting gas indicates that silicon has been depleted into grains.

Everywhere You Turn

#### Contents #### News [As the Galaxies Turn][1] R. Irion Disks of Destruction R. Irion #### Reviews and Viewpoints Disks Around Stars and the Growth of Planetary Systems J. S. Greaves The Kuiper Belt and the Solar System's Comet Disk B. Gladman From Stars to Dust: Looking into a Circumstellar Disk Through Chondritic Meteorites H. C. Connolly Jr. [Black Hole Accretion][2] R. Narayan and E. Quataert W herever matter concentrates in the vast vacuum of space, chances are you will find a disk. Simple physics makes it so; all you need is a spinning sphere of matter collapsing under its own gravitational pull. As the core contracts, it rotates faster to conserve angular momentum. This spin causes the matter around the core to fall onto the equatorial plane, forming a flattened disk of gas and solids. Angular momentum creeps outward, keeping the core from breaking apart. Meanwhile, disk material moves inward along the equatorial plane, eventually either feeding the core or forming other orbiting objects around the core. That oft-repeated scenario makes disks essential building blocks of the universe. This special section covers the many different flavors of disks in space and what they can tell us about the formation of everything from giant gas planets to galaxies. ![Figure][3] CREDIT: GEMINI OBSERVATORY/GMOS TEAM The solar system got its start when a molecular cloud collapsed to form the Sun and a circumstellar disk of gas and dust. As Connolly notes in his Viewpoint (p. 75), chondritic meteorites contain primordial dust from other nearby stars, evidence that the Sun formed within a cluster of stars. In his Review, Gladman (p. 71) takes us to the edge of the solar system, where remnants of the circumstellar disk are dispersed in the Kuiper belt. The belt is more extensive and more structured than previously thought, and its structure holds clues to planetary formation, planetary migration, possible rogue planets, and the close passage of other stars. Over the past decade, as Greaves describes in her Review (p. 68), the search for circumstellar disks with possible planets around other stars has intensified. Planets around Sunlike stars are now considered to be ubiquitous. Current models of planet formation require a disk full of gas and dust to swirl and sway around the star long enough to accrete giant gas planets. Observations of the different stages of the evolution of circumstellar disks are helping to refine these models. For decades, astrophysicists thought that disk-shaped spiral galaxies turn into featureless balls of stars when they collide with other galaxies. In the first of his two News stories (p. [64][1]), however, Irion describes evidence that the spirals (which include our own Milky Way galaxy) can be surprisingly resilient. Meanwhile, other astronomers are probing how disks of matter pulled from a companion star trigger the nuclear explosions that turn white dwarf stars into type Ia supernovas (p. [66][4]): cosmic flares that help gauge the expansion of the universe. Narayan and Quataert (p. 77) end this special issue with a Review of the most illuminating evidence for black holes: the accretion disks that surround them. As a black hole accretes gas, the gas radiates and provides a thermal signature of the mass, rate of spin, and location of the event horizon of the black hole. This quick tour of disks in space highlights their simplicity and ubiquity. As modeling and observations continue to provide more details of disk complexity, their utility for resolving fundamental mysteries of space, such as planet formation and black hole jets, will grow. [1]: /lookup/doi/10.1126/science.307.5706.64 [2]: /lookup/doi/10.1126/science.1105746 [3]: pending:yes [4]: /lookup/doi/10.1126/science.307.5706.66

Eccentricity Excitation and Apsidal Resonance Capture in the Planetary System υ Andromedae

The orbits of the outer two known planets orbiting υ Andromedae are remarkably eccentric. Planet C possesses an orbital eccentricity of e1 = 0.253. For the more distant planet D, e2 = 0.308. Previous dynamical analyses strongly suggest that the two orbits are nearly coplanar and are trapped in an apsidal resonance in which Δ, the difference between their longitudes of periastron, undergoes a bounded oscillation about 0°. Here we elucidate the origin of these large eccentricities and of the apsidal alignment. Resonant interactions between a remnant circumstellar disk of gas lying exterior to the orbits of both planets can smoothly increase e2. Secular interactions between planets D and C can siphon off the eccentricity of the former to increase that of the latter. Externally amplifying e2 during the phase of the apsidal oscillation when e2/e1 is smallest drives the apsidal oscillation amplitude toward zero. Thus, the substantial eccentricity of planet C and the locking of orbital apsides are both consequences of externally pumping the eccentricity of planet D over timescales exceeding apsidal precession periods of order 104 yr. We explain why the recently detected stellar companion to υ And is largely dynamically decoupled from the planetary system.

Aperture Synthesis 12CO and 13CO Observations of DM Tauri: 350 AU Radius Circumstellar Gas Disk

view Abstract Citations (35) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Aperture Synthesis 12CO and 13CO Observations of DM Tauri: 350 AU Radius Circumstellar Gas Disk Saito, M. ; Kawabe, R. ; Ishiguro, M. ; Miyama, S. M. ; Hayashi, M. ; Handa, T. ; Kitamura, Y. ; Omodaka, T. Abstract We have made aperture synthesis observations of 12CO (1-0), 13CO (1-0), 2.6 mm, and 2.7 mm continuum emissions from a young star DM Tau with the Nobeyama Millimeter Array (NMA). It has been found from our observations that 12CO gas with a size of 5"9 x 4"6 is associated with DM Tau and the systematic velocity gradient of 12CO exists along P.A. = 160°. The observational results suggest that DM Tau is surrounded by the disk structure of molecular gas ("circumstellar gas disk") with a radius of 350 AU and an inclination of 40° and that the kinematics of the disk is consistent with Keplerian rotation around DM Tau with a mass of 0.48 Msun. We have also detected the 13CO, 2.6 mm, and 2.7 mm continuum emissions from the disk around DM Tau. The H2 mass of the gas disk in the outer region (r > 100 AU) is estimated to be 2.3 × 10-3 Msun from the 12CO and 13CO emissions, and that in the inner region (r < 100 AU) is 0.019 Msun from 2.6 mm continuum flux. Although the disk around DM Tau is larger than the solar system, the disk may be a protoplanetary disk from the viewpoint that the mass of the disk is comparable to the minimum mass of the solar nebula (∼0.01 Msun) and the kinematics of the disk is consistent with Keplerian rotation. The radial dependence of H2 gas surface density of the disk is roughly estimated as Σ(r) ∝ r-2.0±0.3 using the outer and the inner masses of the disk, unless the depletion of 13CO occurs in the outer region. This power-law index of 2.0±0.3 is significantly larger than the value of 1.5 adopted in the model of minimum solar nebula (Kyoto model). DM Tau is one of the young stars located in the L1551 star-forming region, including HL Tau, GG Tau, and a protostar, L1551-IRS 5. The systemic velocities of gas disks found around these stars in the different evolutional stages are in the very narrow range (VLSR = 5.7 - 6.4 km s-1), and the major axes of the disks are roughly perpendicular to the direction of the local magnetic field. It is likely that these stars were successively formed in the same parent cloud and that the magnetic field might play an important role in the formation of the star-disk systems. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176398 Bibcode: 1995ApJ...453..384S Keywords: STARS: FORMATION; STARS: PRE-MAIN-SEQUENCE; STARS: CIRCUMSTELLAR MATTER full text sources ADS | data products SIMBAD (21)

A Rotating Gaseous Disk around the T Tauri Star GM Aurigae

We report the detection of a gaseous circumstellar disk around a relatively old T Tauri star, GM Aurigae. Maps at 4″ resolution in λ = 1.4-mm continuum emission and in the 13CO (2 → 1) line reveal unresolved and compact dust and gas associated with the stellar position and at the core of a larger rotating gaseous disk, 950 × 530 AU in extent. The mean velocity gradient across the disk, which is oriented along PA ≈ 50°, is consistent with rotation about an axis at PA = 140°. The structure observed in 13CO aperture synthesis maps agrees well with synthetic maps of the gas emission generated from a model. For a disk that is inclined 30° from face on, in Keplerian rotation, we derive a 0.80M⊙ central mass (star + disk), a systemic velocity, vhel , of 15.38 km sec-1, and a mass, 0.1M⊙. The spectral energy distribution of the dust continuum emission suggests a very similar mass, 0.09M⊙.

Magnetic field in a low-mass protostar disk - Millimeter polarimetry of IRAS 16293-2422

We have measured the millimeter polarization of IRAS 16293-2422, a low-mass protostar candidate embedded in the east streamer of the Rho Ophiuchi dark cloud. The source has a linear polarization at 1.1 mm of (2.8 +/- 0.5) percent at a position angle of 144 deg +/- 5 deg with a 19 arcsec beam. If this polarization is due to thermal emission from magnetically aligned nonspherical grains in a dust disk around the central (proto)star, then the magnetic field threads across the disk at a position angle of 54 deg +/- 5 deg projected on the sky. This direction is the same as that of the minor axis of the elongated circumstellar gas disk (of scale 1600 AU) and of the direction of one of the quadratic outflows. There is no positive evidence for a twisted magnetic field in the dust disk, which constrains the hydromagnetic models for molecular outflows, at least for this source.

OH emission and the nature of Minkowski's footprint (M1-92)

VLA observations of the 1667 and 1612 MHz OH brightness distribution in the bipolar nebula M1-92 are presented. The OH emission originates in the circumstellar gas disk whose axis coincides with that of the bipolar lobes. No OH emission is found in the lobes themselves. A simple axisymmetric model which describes the main features of this emission consists of a fan-shaped disk undergoing pure radial expansion at a velocity of 18 km/s. In this model, the radius of the disk is 2.4 arcsec (1.6 x 10 to the 17th cm) and the opening angle of the fan is 70 deg (FWHM). The bipolar axis is inclined at 35 deg to the line of sight. The mass-loss rate in the disk is estimated to be 5.6 x 10 to the -6th solar mass/yr for an assumed distance of 4.5 kpc, and the kinematic age of the disk is about 3000 yr. The properties of M1-92 are reviewed, including the mass-loss rate, the kinematic age, the IR characteristics, and the radio continuum emission, and all are found to be consistent with the view that M1-92 is a young planetary nebula. 25 refs.

Tomographic imaging of HL Tauri

A two-dimensional image of HL Tauri at 2.2 microns with 0.4-arcsec resolution is constructed from one-dimensional speckle-interferometric scans by means of tomographic back projection. The light distribution is dominated by scattered starlight above and below the midplane of the circumstellar disk of gas and dust. It is suggested that the scattering material is not associated directly with the disk but rather represents particles entrained in gas accreting onto the disk and star. The estimated mass accretion rate of 2 x 10 to the -6th solar mass/yr is in excellent accord with that expected from residual infall associated with the formation of the star. 27 refs.

Observations of gaseous circumstellar disks. III

view Abstract Citations (31) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of Gaseous Circumstellar Disks. III. Hobbs, L. M. Abstract Echelle spectra obtained at the Ca II K line and the Na I D lines are reported for seven stars which show excess fluxes in at least one of the IRAS bands at 25, 60, and 100 microns. Detectable circumstellar absorption lines arising from the gaseous components of the presumed circumstellar dust disks are found in none of these cases, in contrast to the previously reported case of Beta Pic. Conversely, six A or F stars which were known before the launch of IRAS to show gaseous circumstellar disks do not show excess fluxes in any of the IRAS bands. Similar echelle spectra of these stars show Ca II circumstellar absorption lines comparable in four cases to that of Beta Pic. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164556 Bibcode: 1986ApJ...308..854H Keywords: Infrared Spectra; Interstellar Gas; Stellar Envelopes; Stellar Spectra; Absorption Spectra; F Stars; Radiant Flux Density; Spectral Line Width; Astrophysics; STARS: CIRCUMSTELLAR SHELLS full text sources ADS | data products SIMBAD (16)

The incidence of infrared excesses among G-type stars in the direction of the Orion IC association

view Abstract Citations (15) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The incidence infrared excesses among G-type stars in the direction of the Orion IC association. Rydgren, A. E. ; Vrba, F. J. Abstract UBVRI and JHK photometry is presented for 49 late-F to early-K stars in the direction of the Orion Ic association for which Smith, Beckers, and Barden (1983) have measured projected rotational velocities. Four of these stars possess T Tauri-like infrared excesses, presumably due to thermal emission from circumstellar dust. Six other stars show evidence of possible intracluster reddening. The authors find no obvious correlation between stellar luminosity and projected rotational velocity within this sample. Specifically, the slower rotators are not systematically less luminous (smaller inferred mass) than the faster rotators. The available evidence also indicates no physical connection between the gaseous circumstellar disks noted by Smith, Beckers, and Barden around the faster rotators and the presence of T Tauri-like infrared excesses. Publication: The Astronomical Journal Pub Date: March 1984 DOI: 10.1086/113527 Bibcode: 1984AJ.....89..399R Keywords: G Stars; Infrared Astronomy; Late Stars; Orion Constellation; Stellar Spectrophotometry; Angular Velocity; K Stars; Stellar Luminosity; Stellar Rotation; T Tauri Stars; Ubv Spectra; Astrophysics full text sources ADS | data products SIMBAD (53) GCPD (1)

Studies of bipolar nebulae. IV - MZ 3 /= PK 331 - 1 deg 1/

view Abstract Citations (41) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies of bipolar nebulae. IV. Mz 3 (= PK 331-1 1). Cohen, M. ; Fitzgerald, M. P. ; Kunkel, W. ; Lasker, B. M. ; Osmer, P. S. Abstract Optical photography and spectroscopy of the bipolar nebula Mz 3 and its exciting star are combined with infrared observations and preexisting radio data. Elemental abundances are obtained for the bright lobes of this nebula. The nebula is characterized by an electron temperature of 17,000 K and an electron density of 18,000 per cu cm, from the forbidden emission lines, and an overall mean density of 2300 per cu cm. The spectral type of the exciting star is O9.5 or B0. Multiaperture infrared observations and star counts around Mz 3 strongly suggest the existence of a circumstellar dust and gas disk which, in other bipolar nebulae, has only been inferred from the overall morphology. It is suggested that Mz 3 may represent a protoplanetary nebula Publication: The Astrophysical Journal Pub Date: April 1978 DOI: 10.1086/156013 Bibcode: 1978ApJ...221..151C Keywords: Astronomical Spectroscopy; Emission Spectra; Infrared Astronomy; Line Spectra; Nebulae; Abundance; Radio Spectra; Star Distribution; Ubv Spectra; Astronomy; Bipolar Nebulae:Element Abundances; Bipolar Nebulae:Morphology; Bipolar Nebulae:Spectroscopy full text sources ADS | data products SIMBAD (4) Related Materials (5) Part 1: 1975ApJ...196..179C Part 2: 1977ApJ...213...79C Part 4: 1978MNRAS.182..687C Part 5: 1980PASP...92..736C Part 6: 1981PASP...93..288C