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Register a new userCircumstellar disks during various evolutionary stages
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Rene D. Oudmaijer
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Disks are ubiquitous in stellar astronomy, and play a crucial role in the formation and evolution of stars. In this contribution we present an overview of the most recent results, with emphasis on high spatial and spectral resolution. We will start with a general discussion on direct versus indirect detection of disks, and then traverse the HR diagram starting with the pre-Main Sequence and ending with evolved stars.
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We have performed a study of the characteristics of the circumstellar environment of the binary object HD101584, that provides information on a likely evolutionary scenario. We have obtained and analysed ALMA observations, complemented with observations using APEX, of a large number of molecular lines. An analysis of the spectral energy distribution has also been performed. Emissions from 12 molecular species (not counting isotopologues) have been observed, and most of them mapped with angular resolutions in the range 0.1 to 0.6. Four circumstellar components are identified: i) a central compact source of size 0.15, ii) an expanding equatorial density enhancement (a flattened density distribution in the plane of the orbit) of size 3, iii) a bipolar high-velocity outflow (150 km/s), and iv) an hourglass structure. The outflow is directed almost along the line of sight. There is evidence of a second bipolar outflow. The mass of the circumstellar gas is 0.5[D/1 kpc]^2 Msun, about half of it lies in the equatorial density enhancement. The dust mass is 0.01[D/1 kpc]^2 Msun, and a substantial fraction of this is in the form of large-sized, up to 1 mm, grains. The estimated kinetic age of the outflow is 770[D/1 kpc] yr. The kinetic energy and the scalar momentum of the accelerated gas are estimated to be 7x10^(45)[D/1 kpc]^2 erg and 10^(39)[D/1 kpc]^2 g cm/s, respectively. We provide good evidence that the binary system HD101584 is in a post-common-envelope-evolution phase, that ended before a stellar merger. Isotope ratios combined with stellar mass estimates suggest that the primary stars evolution was terminated already on the first red giant branch (RGB). Most of the energy required to drive the outflowing gas was probably released when material fell towards the companion.
In this paper, we present a multi-wavelength analysis of an eruptive white-light M3.2 flare which occurred in active region NOAA 10486 on November 1, 2003. Excellent set of high resolution observations made by RHESSI and TRACE provide clear evidence of significant pre-flare activities for ~9 minutes in the form of an initiation phase observed at EUV/UV wavelengths followed by the X-ray precursor phase. During the initiation phase, we observed localized brightenings in the highly sheared core region close to the filament and interactions among short EUV loops overlying the filament which led to the opening of magnetic field lines. The X-ray precursor phase is manifested in RHESSI measurements below ~30 keV and coincided with the beginning of flux emergence at the flaring location along with early signatures of the eruption. From the RHESSI observations, we conclude that both plasma heating and electron acceleration occurred during the precursor phase. The main flare is consistent with the standard flare model. However, after the impulsive phase, intense HXR looptop source was observed without significant footpoint emission. More intriguingly, for a brief period the looptop source exhibited strong HXR emission with energies up to 100 keV and significant non-thermal characteristics. The present study indicates a causal relation between the activities in the preflare and main flare. We also conclude that pre-flare activities, occurred in the form of subtle magnetic reorganization along with localized magnetic reconnection, played a crucial role in destabilizing the active region filament leading to solar eruptive flare and associated large-scale phenomena.
[Abridged] Star and planet formation are the complex outcomes of gravitational collapse and angular momentum transport mediated by protostellar and protoplanetary disks. In this review we focus on the role of gravitational instability in this process. We begin with a brief overview of the observational evidence for massive disks that might be subject to gravitational instability, and then highlight the diverse ways in which the instability manifests itself in protostellar and protoplanetary disks: the generation of spiral arms, small scale turbulence-like density fluctuations, and fragmentation of the disk itself. We present the analytic theory that describes the linear growth phase of the instability, supplemented with a survey of numerical simulations that aim to capture the non-linear evolution. We emphasize the role of thermodynamics and large scale infall in controlling the outcome of the instability. Despite apparent controversies in the literature, we show a remarkable level of agreement between analytic predictions and numerical results. We highlight open questions related to (1) the development of a turbulent cascade in thin disks, and (2) the role of mode-mode coupling in setting the maximum angular momentum transport rate in thick disks.
We present medium-resolution K-band long-slit spectroscopy of 29 true, likely, possible and candidate Galactic Plane planetary nebulae (PNe) from the UWISH2 survey - many of which have only been recently discovered. These objects are bright in molecular hydrogen (H2) emission, and many have bipolar morphologies. Through the detection of the Br{gamma} emission line, which traces ionized hydrogen, we find that the majority of the candidate PNe are indeed likely to be PNe, while 2 of the targets are more likely young stellar objects (YSOs) or pre-planetary nebulae (pPNe). We detect Br{gamma} in 13 objects which have no detection in IPHAS or SHS H{alpha} surveys. This implies they are potential members of the little-known optically-obscured PN population, hidden from wide-field optical surveys. We use the spatial extent of the H2 1-0 S(1) and Br{gamma} lines to estimate the evolutionary stage of our targets, and find that W-BPNe (bipolar PNe with pinched waist morphologies) are likely to be younger objects, while R-BPNe (bipolar PNe with large ring structures) are more evolved. We use line ratios to trace the excitation mechanism of the H2, and find the 1-0 S(1) / 2-1 S(1) and 1-0 S(1) / Br{gamma} ratios are higher for R-BPNe, implying the H2 is thermally excited. However, in W-BPNe, these ratios are lower, and so UV-fluorescence may be contributing to the excitation of H2.
We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex and one, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of h2 to study the evolution of the gas in the inner disk. We find that the h2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting h2 in the disk, the fact that the non-accreting sources show no h2 emission implies that the h2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 {AA}, we estimate that the column density of h2 left in the inner regions of the debris disks in our sample is less than ~ 3x10^-6 g cm^-2, nine orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU.
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