Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userH2 and CO emission from disks around TTauri and HerbigAe pre-main-sequence stars and from debris disks around young stars: warm and cold circumstellar gas
60
0
0.0
(
0
)
Authors
W. F. Thi
Ask ChatGPT about the research
No Arabic abstract
We present ISO-SWS observations of H2 pure-rotational line emission from the disks around low and intermediate mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. We detect `warm (T ~ 100-200 K) H2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~1E-4 Solar mass up to 8E-3 Solar mass, and can constitute a non-negligible fraction of the total disk mass. Complementary single-dish 12CO 3-2, 13CO 3-2 and 12CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T ~ 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Keplerian rotation, consistent with interferometer data on the lower-J lines. The ratios of the 12CO 3-2/ 13CO 3-2 integrated fluxes indicate that 12CO 3-2 is optically thick but that 13CO 3-2 is optically thin or at most moderately thick. The 13CO 3-2 lines have been used to estimate the cold gas mass. If a H2/CO conversion factor of 1E4 is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100,in accordance with previous studies. The warm gas is typically 1-10 % of the total mass deduced from millimeter continuum emission, but can increase up to 100% or more for the debris-disk objects. Thus, residual molecular gas may persist into the debris-disk phase. No significant evolution in the H2, CO or dust masses is found for stars with ages in the range of 1E6-1E7 years, although a decrease is found for the older debris-disk star beta Pictoris. Existing models fail to explain the amount of warm gas quantitatively.
rate research
Read More
The results of single-dish observations of low- and high-J transitions of selected molecules from protoplanetary disks around two TTauri stars (LkCa15 and TWHya) and two HerbigAe stars (HD163296 and MWC480) are reported. Simple molecules such as CO, 13CO, HCO+, CN and HCN are detected. Several lines of H2CO are found toward the TTauri star LkCa15 but not in other objects. No CH3OH has been detected down to abundances of 10E-9 - 10E-8 with respect to H2. SO and CS lines have been searched for without success. Line ratios indicate that the molecular emission arises from dense 10E6 - 10E8 cm-3 and moderately warm (T ~ 20-40K) intermediate height regions of the disk atmosphere, in accordance with predictions from models of the chemistry in disks. The abundances of most species are lower than in the envelope around the solar-mass protostar IRAS 16293-2422. Freeze-out and photodissociation are likely causes of the depletion. DCO+ is detected toward TWHya, but not in other objects. The high inferred DCO+/HCO+ ratio of ~0.035 is consistent with models of the deuterium fractionation in disks which include strong depletion of CO. The inferred ionization fraction in the intermediate height regions as deduced from HCO+ is at least 10E-11 - 10E-10, comparable to that derived for the midplane from recent H2D+ observations. (abridged abstract)
We conducted a survey for infrared excess emission from 16 nearby main sequence shell stars using the Multiband Imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope. Shell stars are early-type stars with narrow absorption lines in their spectra that appear to arise from circumstellar (CS) gas. Four of the 16 stars in our survey showed excess emission at 24 microns and 70 microns characteristic of cool CS dust and are likely to be edge-on debris disks. Including previously known disks, it appears that the fraction of protoplanetary and debris disks among the main sequence shell stars is at least 48% +/- 14%. While dust in debris disks has been extensively studied, relatively little is known about their gas content. In the case of Beta Pictoris, extensive observations of gaseous species have provided insights into the dynamics of the CS material and surprises about the composition of the CS gas coming from young planetesimals (e.g. Roberge et al. 2006). To understand the co-evolution of gas and dust through the terrestrial planet formation phase, we need to study the gas in additional debris disks. The new debris disk candidates from this Spitzer survey double the number of systems in which the gas can be observed right now with sensitive line of sight absorption spectroscopy.
We calculate the emission of protoplanetary disks threaded by a poloidal magnetic field and irradiated by the central star. The radial structure of these disks was studied by Shu and collaborators and the vertical structure was studied by Lizano and collaborators. We consider disks around low mass protostars, T Tauri stars, and FU Ori stars with different mass-to-flux ratios $lambda_{rm sys}$. We calculate the spectral energy distribution and the antenna temperature profiles at 1 mm and 7 mm convolved with the ALMA and VLA beams. We find that disks with weaker magnetization (high values of $lambda_{rm sys}$) emit more than disks with stronger magnetization (low values of $lambda_{rm sys}$). This happens because the former are denser, hotter and have larger aspect ratios, receiving more irradiation from the central star. The level of magnetization also affects the optical depth at millimeter wavelengths, being larger for disks with high $lambda_{rm sys}$. In general, disks around low mass protostars and T Tauri stars are optically thin at 7 mm while disks around FU Ori are optically thick. A qualitative comparison of the emission of these magnetized disks, including heating by an external envelope, with the observed millimeter antenna temperature profiles of HL Tau indicates that large cm grains are required to increase the optical depth and reproduce the observed 7 mm emission at large radii.
We present photometric ISO 60 and 170um measurements, complemented by some IRAS data at 60um, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Of the stars younger than 400 Myr one in two has a disk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their disk in a similar way: some stars as old as the Sun still have significant disks.
According to the current paradigm of circumstellar disk evolution, gas-rich primordial disks evolve into gas-poor debris disks compose of second-generation dust. To explore the transition between these phases, we searched for $^{12}$CO, $^{13}$CO, and C$^{18}$O emission in seven dust-rich debris disks around young A-type stars, using ALMA in Band 6. We discovered molecular gas in three debris disks. In all these disks, the $^{12}$CO line was optically thick, highlighting the importance of less abundant molecules in reliable mass estimates. Supplementing our target list by literature data, we compiled a volume-limited sample of dust-rich debris disks around young A-type stars within 150 pc. We obtained a CO detection rate of 11/16 above a $^{12}$CO J=2$-$1 line luminosity threshold of $sim 1.4 times 10 ^4$ Jykms$^{-1}$pc$^2$ in the sample. This high incidence implies that the presence of CO gas in bright debris disks around young A-type stars is likely more the rule than the exception. Interestingly, dust-rich debris disks around young FG-type stars exhibit, with the same detectability threshold as for A-type stars, significantly lower gas incidence. While the transition from protoplanetary to debris phase is associated with a drop of dust content, our results exhibit a large spread in the CO mass in our debris sample, with peak values comparable to those in protoplanetary Herbig Ae disks. In the particularly CO-rich debris systems the gas may have primordial origin, characteristic of a hybrid disk.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
