ترغب بنشر مسار تعليمي؟ اضغط هنا

Spectrally resolved pure rotational lines of water in protoplanetary disks

148   0   0.0 ( 0 )
 نشر من قبل Klaus Martin Pontoppidan
 تاريخ النشر 2010
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

We present ground-based high resolution N-band spectra (Delta v = 15 km/s) of pure rotational lines of water vapor in two protoplanetary disks surrounding the pre-main sequence stars AS 205N and RNO 90, selected based on detections of rotational water lines by the Spitzer IRS. Using VISIR on the Very Large Telescope, we spectrally resolve individual lines and show that they have widths of 30-60 km/s, consistent with an origin in Keplerian disks at radii of ~1 AU. The water lines have similar widths to those of the CO at 4.67 micron, indicating that the mid-infrared water lines trace similar radii. The rotational temperatures of the water are 540 and 600K in the two disks, respectively. However, the lines ratios show evidence of non-LTE excitation, with low-excitation line fluxes being over-predicted by 2-dimensional disk LTE models. Due to the limited number of observed lines and the non-LTE line ratios, an accurate measure of the water ortho/para ratio is not available, but a best estimate for AS 205N is ortho/para = 4.5 +/- 1.0, apparently ruling out a low-temperature origin of the water. The spectra demonstrate that high resolution spectroscopy of rotational water lines is feasible from the ground, and further that ground-based high resolution spectroscopy is likely to significantly improve our understanding of the inner disk chemistry recently revealed by recent Spitzer observations.



قيم البحث

اقرأ أيضاً

Circumstellar disks provide the material reservoir for the growth of young stars and for planet formation. We combine a high-level radiative transfer program with a thermal-chemical model of a typical T Tauri star disk to investigate the diagnostic p otential of the far-infrared lines of water for probing disk structure. We discuss the observability of pure rotational H2O lines with the Herschel Space Observatory, specifically the residual gas where water is mainly frozen out. We find that measuring both the line profile of the ground 110-101 ortho-H2O transition and the ratio of this line to the 312-303 and 221-212 line can provide information on the gas phase water between 5-100 AU, but not on the snow line which is expected to occur at smaller radii.
We report on a limited search for pure-rotational molecular hydrogen emission associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1 and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle Spectrograph (TEXES) at NASAs 3m Infrared Telescope Facility. The high spectral and spatial resolution of our observations lead to more stringent limits on narrow line emission close to the source than previously achieved. One star, AB Aur, shows a possible (2sigma) H_2 detection, but further observations are required to make a confident statement. Our non-detections suggest that a significant fraction, perhaps all, of previously reported H_2 emission towards these objects could be extended on scales of 5 or more.
We performed very deep searches for 2 ground-state water transitions in 13 protoplanetary disks with the HIFI instrument on-board the Herschel Space Observatory, with integration times up to 12 hours per line. Two other water transitions that sample warmer gas were also searched for with shallower integrations. The detection rate is low, and the upper limits provided by the observations are generally much lower than predictions of thermo-chemical models with canonical inputs. One ground-state transition is newly detected in the stacked spectrum of AA Tau, DM Tau, LkCa 15, and MWC 480. We run a grid of models to show that the abundance of gas-phase oxygen needs to be reduced by a factor of at least ~100 to be consistent with the observational upper limits (and positive detections) if a dust-to-gas mass ratio of 0.01 were to be assumed. As a continuation of previous ideas, we propose that the underlying reason for the depletion of oxygen (hence the low detection rate) is the freeze-out of volatiles such as water and CO onto dust grains followed by grain growth and settling/migration, which permanently removes these gas-phase molecules from the emissive upper layers of the outer disk. Such depletion of volatiles is likely ubiquitous among different disks, though not necessarily to the same degree. The volatiles might be returned back to the gas phase in the inner disk (within about 15 AU), which is consistent with current constraints. Comparison with studies on disk dispersal due to photoevaporation indicates that the timescale for volatile depletion is shorter than that of photoevaporation.
The low water content of the terrestrial planets in the solar system suggests that the protoplanets formed within the water snow line. Accurate prediction of the snow line location moving with time provides a clue to constrain the formation process o f the planets. In this paper, we investigate the migration of the snow line in protoplanetary disks whose accretion is controlled by laminar magnetic fields, which have been proposed by various nonideal magnetohydrodynamic (MHD) simulations. We propose an empirical model of the disk temperature based on our nonideal MHD simulations, which show that the accretion heating is significantly less efficient than in turbulent disks, and calculate the snow line location over time. We find that the snow line in the magnetically accreting laminar disks moves inside the current Earths orbit within 1 Myr after star formation, whereas the time for the conventional turbulent disk is much longer than 1 Myr. This result suggests that either the rocky protoplanets formed in such an early phase of the disk evolution, or the protoplanets moved outward to the current orbits after they formed close to the protosun.
Mid-IR water lines from protoplanetary disks around T Tauri stars have a detection rate of 50%. Models have identified multiple physical properties of disks such as dust-to-gas mass ratio, dust size power law distribution, disk gas mass, disk inner r adius, and disk scale height as potential explanation for the current detection rate. We search for a connection between mid-IR water line fluxes and the strength of the 10~$mu$m silicate feature. We analyse observed water line fluxes from three blends and compute the 10~$mu$m silicate feature strength from Spitzer spectra. We use a series of published models, exploring disk dust and gas properties, and the effects of different stars. The models also show that the increasing stellar luminosity enhance simultaneously the strength of this dust feature and the water lines fluxes. No correlation is found between the observed mid-IR water lines and the 10~$mu$m silicate. Our sample shows the same difference in the peak strength between amorphous and crystalline silicates that was noted in earlier studies, but our models do not support this intrinsic difference in silicate peak strength. Individual properties of our model series are not able to reproduce the most extreme observations, suggesting that more complex dust properties are required. A parametrized settling prescription is able to boost the peak strength by a factor 2 for the standard model. Water line fluxes are unrelated to the composition of the dust. The pronounced regular trends seen in the model results are washed out in the data due to the larger diversity in stellar and disk properties compared to our model series. The disks with with weaker mid-IR water line fluxes are depleted in gas or enhanced in dust in the inner 10~au. In the case of gas depleted disks, settling produces very strong 10~$mu$m silicate features, with strong peak strength.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا