Gas signatures of Herbig Ae/Be disks probed with Herschel SPIRE spectroscopy

Herbig Ae/Be objects, like their lower mass counterparts T Tauri stars, are seen to form a stable circumstellar disk which is initially gas-rich and could ultimately form a planetary system. We present Herschel SPIRE 460-1540 GHz spectra of five targets out of a sample of 13 young disk sources, showing line detections mainly due to warm CO gas.Comment: to be published in proceedings of IAU symposium 299 (Victoria, BC, Canada, June 2013

Hydrogen Fluoride in High-Mass Star-forming Regions

Hydrogen fluoride has been established to be an excellent tracer of molecular hydrogen in diffuse clouds. In denser environments, however, the HF abundance has been shown to be approximately two orders of magnitude lower. We present Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption in foreground clouds and the source itself, while in AFGL 2591 HF is partially in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds are most likely caused by freeze out of HF molecules onto dust grains in high-density gas. In AFGL 2591, the view of the hot core is obstructed by absorption in the massive outflow, in which HF is also very abundant 3.6e-8) due to the desorption by sputtering. These observations provide further evidence that the chemistry of interstellar fluorine is controlled by freeze out onto gas grains.Comment: accepted in Ap

Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A

Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed. We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, $HCO^{+}$, HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in LTE as well as through more detailed non-LTE radiative transfer calculations. Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO$^{+}$, HCN, and SO are also detected from the inner 100 au region. From the derived $HCO^{+}$ abundance, we estimate the ionization fraction of the disk surface and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk's molecular abundances relative to Solar System objects. Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and $H_2 O$ molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation.Comment: Accepted for publication on A&A; 15 pages, 13 figure

The Herschel/SPIRE Spectrometer Useful Scripts

In most cases, the Standard Product Generation (SPG) processing pipelines for the Herschel SPIRE Fourier Transform Spectrometer (FTS) produce well-calibrated spectra of high quality. However, some Astronomical sources, such as those with a faint continuum, require additional processing to achieve more meaningful spectra. In consultation with the astronomical community, a set of scripts were developed to assist in the post-pipeline analysis of the spectra

Evidence for the start of planet formation in a young circumstellar disk

The growth of dust grains in protoplanetary disks is a necessary first step towards planet formation. This growth has been inferred via observations of thermal dust emission towards mature protoplanetary systems (age >2 million years) with masses that are, on average, similar to Neptune3. In contrast, the majority of confirmed exoplanets are heavier than Neptune. Given that young protoplanetary disks are more massive than their mature counterparts, this suggests that planet formation starts early, but evidence for grain growth that is spatially and temporally coincident with a massive reservoir in young disks remains scarce. Here, we report observations on a lack of emission of carbon monoxide isotopologues within the inner ~15 au of a very young (age ~100,000 years) disk around the Solar-type protostar TMC1A. By using the absence of spatially resolved molecular line emission to infer the gas and dust content of the disk, we conclude that shielding by millimeter-size grains is responsible for the lack of emission. This suggests that grain growth and millimeter-size dust grains can be spatially and temporally coincident with a mass reservoir sufficient for giant planet formation. Hence, planet formation starts during the earliest, embedded phases in the life of young stars.Comment: Accepted for publication in Nature Astronomy, 3 figures, 3 extended figure

Herschel SPIRE FTS Spectral Mapping Calibration

The Herschel SPIRE Fourier transform spectrometer (FTS) performs spectral imaging in the 447-1546 GHz band. It can observe in three spatial sampling modes: sparse mode, with a single pointing on sky, or intermediate or full modes with 1 and 1/2 beam spacing, respectively. In this paper, we investigate the uncertainty and repeatability for fully sampled FTS mapping observations. The repeatability is characterised using nine observations of the Orion Bar. Metrics are derived based on the ratio of the measured intensity in each observation compared to that in the combined spectral cube from all observations. The mean relative deviation is determined to be within 2%, and the pixel-by-pixel scatter is ~7%. The scatter increases towards the edges of the maps. The uncertainty in the frequency scale is also studied, and the spread in the line centre velocity across the maps is found to be ~15 km/s. Other causes of uncertainty are also discussed including the effect of pointing and the additive uncertainty in the continuum.Comment: 12 pages, 9 figures, accepted for publication in Experimental Astronom