L. Podio

GASPS—A Herschel survey of gas and dust in protoplanetary disks: summary and initial statistics

We describe a large-scale far-infrared line and continuum survey of protoplanetary disk through to young debris disk systems carried out using the ACS instrument on the Herschel Space Observatory. This Open Time Key program, known as GASPS (Gas Survey of Protoplanetary Systems), targeted similar to 250 young stars in narrow wavelength regions covering the [OI] fine structure line at 63 mu m the brightest far-infrared line in such objects. A subset of the brightest targets were also surveyed in [OI]145 mu m, [CII] at 157 mu m, as well as several transitions of H2O and high-excitation CO lines at selected wavelengths between 78 and 180 mu m. Additionally, GASPS included continuum photometry at 70, 100 and 160 mu m, around the peak of the dust emission. The targets were SED Class II-III T Tauri stars and debris disks from seven nearby young associations, along with a comparable sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide sample of circumstellar disks, combining the results with models in a systematic way. In this overview paper we review the scientific aims, target selection and observing strategy of the program. We summarise some of the initial results, showing line identifications, listing the detections, and giving a first statistical study of line detectability. The [OI] line at 63 mu m was the brightest line seen in almost all objects, by a factor of similar to 10. Overall [OI]63 mu m detection rates were 49%, with 100% of HAeBe stars and 43% of T Tauri stars detected. A comparison with published disk dust masses (derived mainly from sub-mm continuum, assuming standard values of the mm mass opacity) shows a dust mass threshold for [OI] 63 mu m detection of similar to 10(-5) M-circle dot. Normalising to a distance of 140 pc, 84% of objects with dust masses >= 10(-5) M-circle dot can be detected in this line in the present survey; 32% of those of mass 10(-6)-10(-5) M-circle dot, and only a very small number of unusual objects with lower masses can be detected. This is consistent with models with a moderate UV excess and disk flaring. For a given disk mass, [OI] detectability is lower for M stars compared with earlier spectral types. Both the continuum and line emission was, in most systems, spatially and spectrally unresolved and centred on the star, suggesting that emission in most cases was from the disk. Approximately 10 objects showed resolved emission, most likely from outflows. In the GASPS sample, [OI] detection rates in T Tauri associations in the 0.3-4 Myr age range were similar to 50%. For each association in the 5-20 Myr age range, similar to 2 stars remain detectable in [OI]63 mu m, and no systems were detected in associations with age >20 Myr. Comparing with the total number of young stars in each association, and assuming a ISM-like gas/dust ratio, this indicates that similar to 18% of stars retain a gas-rich disk of total mass similar to 1 M-Jupiter for 1-4 Myr, 1-7% keep such disks for 5-10 Myr, but none are detected beyond 10-20 Myr. The brightest [OI] objects from GASPS were also observed in [OI]145 mu m, [CII]157 mu m and CO J = 18 - 17, with detection rates of 20-40%. Detection of the [CII] line was not correlated with disk mass, suggesting it arises more commonly from a compact remnant envelope.

Herschel/PACS Survey of Protoplanetary Disks in Taurus/Auriga—Observations of [O I] and [C II], and Far-infrared Continuum

TheHerschelSpaceObservatory was used to observe ∼120 pre-main-sequence stars in Taurus as part of the GASPS Open Time Key project. Photodetector Array Camera and Spectrometer was used to measure the continuum as well as several gas tracers such as [Oi ]6 3μm, [Oi] 145 μm, [Cii] 158 μm, OH, H2O, and CO. The strongest line seen is [Oi ]a t 63μm. We find a clear correlation between the strength of the [Oi ]6 3μm line and the 63 μm continuum for disk sources. In outflow sources, the line emission can be up to 20 times stronger than in disk sources, suggesting that the line emission is dominated by the outflow. The tight correlation seen for disk sources suggests that the emission arises from the inner disk (<50 AU) and lower surface layers of the disk where the gas and dust are coupled. The [Oi ]6 3μm is fainter in transitional stars than in normal Class II disks. Simple spectral energy distribution models indicate that the dust responsible for the continuum emission is colder in these disks, leading to weaker line emission. [Cii] 158 μm emission is only detected in strong outflow sources. The observed line ratios of [Oi ]6 3μ mt o [Oi] 145 μm are in the regime where we are insensitive to the gas-to-dust ratio, neither can we discriminate between shock or photodissociation region emission. We detect no Class III object in [Oi ]6 3μm and only three in continuum, at least one of which is a candidate debris disk.

IR diagnostics of embedded jets: velocity resolved observations of the HH34 and HH1 jets

Context. We present VLT-ISAAC medium resolution spectroscopy of the HH34 and HH1 jets, driven by young Class 0/I sources. Aims. Our aim is to derive the kinematics and the physical parameters associated with infrared jets, and to study how they vary with jet velocity. Methods. We use several important diagnostic lines covered by our spectral range, such as [Feii ]1 .644 μm, 1.600 μ ma nd H2 2.122 μm, to probe both the atomic and the molecular jet components. Results. In the inner jet region of HH34, we find that both the atomic and molecular gas present two components at high and low velocity (the so-called HVC and LVC), as reported by previous studies. The [Fe ii] LVC in HH34 is detected up to large distances from the source (>1000 AU), at variance with TTauri jets where the LVC is usually confined within 200 AU from the star. In H2 2.122 μm, the LVC and HVC are spatially separated, with an abrupt transition from low- to high-velocity emission at ∼1. �� 5. We moreover detect, for the first time, the fainter red-shifted counterpart down to the central source. This lobe shows several emission knots displaced symmetrically with respect to the corresponding blue-shifted gas. In HH1, we trace the jet down to ∼1 �� from the VLA1 driving source: the kinematics of this inner region is again characterised by the presence of two velocity components, one blue-shifted and one red-shifted with respect to the source LSR velocity. We interpret this double component as arising from the interaction of two different jets. We suggest that the red-shifted component could be part of the HH501 jet. Electron densities and mass fluxes have been measured separately for the different velocity components in the HH34 and HH1 jets. In the inner HH34 jet region, ne increases with decreasing velocity, with an average value of ∼1 × 10 4 cm −3 in the HVC and ∼2.2 × 10 4 cm −3 in the LVC. Up to ∼10 �� from the driving source, and along the whole HH1 jet an opposite behaviour is observed instead, with ne increasing with velocity. In both jets t

Herschel/PACS observations of young sources in Taurus: the far-infrared counterpart of optical jets

Context. Observations of the atomic and molecular line emission associated with jets and outflows emitted by young stellar objects provide sensitive diagnostics of the excitation conditions, and can be used to trace the various evolutionary stages they pass through as they evolve to become main sequence stars. Aims. To understand the relevance of atomic and molecular cooling in shocks, and how accretion and ejection efficiency evolves with the evolutionary state of the sources, we will study the far-infrared counterparts of bright optical jets associated with Class I and II sources in Taurus (T Tau, DG Tau A, DG Tau B, FS Tau A+B, and RW Aur). Methods. We have analysed Herschel/PACS observations of a number of atomic ([O i]63 μm, 145 μm, [C ii]158 μm) and molecular (high-J CO, H2O, OH) lines, collected within the open time key project GASPS (PI: W. R. F. Dent). To constrain the origin of the detected lines we have compared the obtained FIR emission maps with the emission from optical-jets and millimetre-outflows, and the measured line fluxes and ratios with predictions from shock and disk models. Results. All of the targets are associated with extended emission in the atomic lines; in particular, the strong [O i ]6 3μm emission is correlated with the direction of the optical jet/mm-outflow. The line ratios suggest that the atomic lines can be excited in fast dissociative J-shocks occurring along the jet. The molecular emission, on the contrary, originates from a compact region, that is spatially and spectrally unresolved, and lines from highly excited levels are detected (e.g., the o-H2 O8 18–707 line, and the CO J = 36−35 line). Disk models are unable to explain the brightness of the observed lines (CO and H2O line fluxes up to 10 −15 −6 × 10 −16 Wm −2 ). Slow C- or J-shocks with high pre-shock densities reproduce the observed H2O and high-J CO lines; however, the disk and/or UV-heated outflow cavities may contribute to the observed emission. Conclusions. Similarly to Class 0 sources, the FIR emission associated with Class I and II jet-sources is likely to be shock-excited. While the cooling is dominated by CO and H2O lines in Class 0 sources, [O i] becomes an important coolant as the source evolves and the environment is cleared. The cooling and mass loss rates estimated for Class II and I sources are one to four orders of magnitude lower than for Class 0 sources. This provides strong evidence to indicate that the outflow activity decreases as the source evolves.

The outflow history of two Herbig-Haro jets in RCW 36: HH 1042 and HH 1043

Jets around low- and intermediate-mass young stellar objects (YSOs) contain a fossil record of the recent accretion and outflow activity of their parent star-forming systems. We aim to understand whether the accretion/ejection process is similar across the entire stellar mass range of the parent YSOs. To this end we have obtained optical to near-infrared spectra of HH 1042 and HH 1043, two newly discovered jets in the massive star-forming region RCW 36, using X-shooter on the ESO Very Large Telescope. HH 1042 is associated with the intermediate-mass YSO 08576nr292. Over 90 emission lines are detected in the spectra of both targets. Highvelocity (up to 220 km s 1 ) blue- and redshifted emission from a bipolar flow is observed in typical shock tracers. Low-velocity emission from the background cloud is detected in nebular tracers, including lines from high ionization species. We applied combined optical and infrared spectral diagnostic tools in order to derive the physical conditions (density, temperature, and ionization) in the jets. The measured mass outflow rates are ˙ Mjet 10 7 M yr 1 . It is not possible to determine a reliable estimate for the accretion rate of the driving source of HH 1043 using optical tracers. We measure a high accretion rate for the driving source of HH 1042 ( ˙ Macc 10 6 M yr 1 ). For this system the ratio ˙ Mjet= ˙ Macc 0:1, which is comparable to low-mass sources and consistent with models for magneto-centrifugal jet launching. The knotted structure and velocity spread in both jets are interpreted as fossil signatures of a variable outflow rate. While the mean velocities in both lobes of the jets are comparable, the variations in mass outflow rate and velocity in the two lobes are not symmetric. This asymmetry suggests that the launching mechanism on either side of the accretion disk is not synchronized. For the HH 1042 jet, we have constructed an interpretative physical model with a stochastic or periodic outflow rate and a description of a ballistic flow as its constituents. We have simulated the flow and the resulting emission in position velocity space, which is then compared to the observed kinematic structure. The knotted structure and velocity spread can be reproduced qualitatively with the model. The results of the simulation indicate that the outflow velocity varies on timescales on the order of 100 yr.

Molecular ions in the protostellar shock L1157-B1

Aims. We perform a complete census of molecular ions with an abundance greater than ∼10 −10 in the protostellar shock L1157-B1. This allows us to study the ionisation structure and chemistry of the shock. Methods. An unbiased high-sensitivity survey of L1157-B1 performed with the IRAM-30 m and Herschel/HIFI as part of the CHESS and ASAI large programmes allows searching for molecular ions emission. Then, by means of a radiative transfer code in the large velocity gradient approximation, the gas physical conditions and fractional abundances of molecular ions are derived. The latter are compared with estimates of steady-state abundances in the cloud and their evolution in the shock calculated with the chemical model Astrochem. Results. We detect emission from HCO + ,H 13 CO + ,N 2H + ,H CS + , and for the first time in a shock, from HOCO + and SO + . The bulk of the emission peaks at blue-shifted velocity, ∼0.5‐3 km s −1 with respect to systemic, has a width of ∼3‐7 km s −1 and is associated with the outflow cavities (Tkin ∼ 20−70 K, nH2 ∼ 10 5 cm −3 ). A high-velocity component up to −40 km s −1 , associated with the primary jet, is detected in the HCO + 1‐0 line. Observed HCO + and N2H + abundances (XHCO+ ∼ 0.7−3 × 10 −8 , XN2H+ ∼ 0.4−8 × 10 −9 ) agree with steady-state abundances in the cloud and with their evolution in the compressed and heated gas in the shock for cosmic rays ionisation rate ζ = 3 × 10 −16 s −1 . HOCO + ,S O + ,a nd HCS + observed abundances (XHOCO+ ∼ 10 −9 , XSO+ ∼ 8 × 10 −10 , XHCS+ ∼ 3−7 × 10 −10 ), instead, are 1‐2 orders of magnitude larger than predicted in the cloud; on the other hand, they are strongly enhanced on timescales shorter than the shock age (∼2000 years) if CO2 ,So r H 2S, and OCS are sputtered off the dust grains in the shock. Conclusions. The performed analysis indicates that HCO + and N2H + are a fossil record of pre-shock gas in the outflow cavity, whilst HOCO + ,S O + ,a nd HCS + are effective shock tracers that can be used to infer the amount of CO2 and sulphur-bearing species released from dust mantles in the shock. The observed HCS + (and CS) abundance indicates that OCS should be one of the main sulphur carrier on grain mantles. However, the OCS abundance required to fit the observations is 1‐2 orders of magnitude larger than observed. Laboratory experiments are required to measure the reactions rates involving these species and to fully understand the chemistry of sulphur-bearing species.

The jet and the disk of the HH 212 low-mass protostar imaged by ALMA: SO and SO2 emission

Context. The investigation of the disk formation and jet launching mechanism in protostars is crucial to understanding the earliest stages of star and planet formation. Aims. We aim to constrain the physical and dynamical properties of the molecular jet and disk of the HH 212 protostellar system at unprecedented angular scales, exploiting the capabilities of the Atacama Large Millimeter Array (ALMA). Methods. The ALMA observations of HH 212 in emission lines from sulfur-bearing molecules, SO 98−87 ,S O 10 11−1010, SO2 82,6−71,7, are compared with simultaneous CO 3−2, SiO 8−7 data. The molecules column density and abundance are estimated using simple radiative transfer models. Results. SO 98−87 and SO2 82,6−71,7 show broad velocity profiles. At systemic velocity, they probe the circumstellar gas and the cavity walls. Going from low to high blue- and red-shifted velocities the emission traces the wide-angle outflow and the fast (∼100−200 km s −1 ), collimated (∼90 AU) molecular jet revealing the inner knots with timescales ≤50 yr. The jet transports a mass− −−

Water Vapor in the Protoplanetary Disk of DG Tau

Water is key in the evolution of protoplanetary disks and the formation of comets and icy/water planets. While high excitation water lines originating in the hot inner disk have been detected in several T Tauri stars (TTSs), water vapor from the outer disk, where most of water ice reservoir is stored, was only reported in the closeby TTS TW Hya. We present spectrally resolved Herschel/HIFI observations of the young TTS DG Tau in the ortho- and para- water ground-state transitions at 557, 1113 GHz. The lines show a narrow double-peaked profile, consistent with an origin in the outer disk, and are 19-26 times brighter than in TW Hya. In contrast, CO and [C II] lines are dominated by emission from the envelope/outflow, which makes H2O lines a unique tracer of the disk of DG Tau. Disk modeling with the thermo-chemical code ProDiMo indicates that the strong UV field, due to the young age and strong accretion of DG Tau, irradiates a disk upper layer at 10-90 AU from the star, heating it up to temperatures of 600 K and producing the observed bright water lines. The models suggest a disk mass of 0.015-0.1 Msun, consistent with the estimated minimum mass of the solar nebula before planet formation, and a water reservoir of 1e2-1e3 Earth oceans in vapour, and 100 times larger in the form of ice. Hence, this detection supports the scenario of ocean delivery on terrestrial planets by impact of icy bodies forming in the outer disk.

Jet multiplicity in the proto-binary system NGC 1333-IRAS4A

Context. Owing to the paucity of sub-arcsecond (sub)mm observations required to probe the innermost regions of newly forming protostars, several fundamental questions are still being debated, such as the existence and coevality of close multiple systems. Aims. We study the physical and chemical properties of the jets and protostellar sources in the NGC 1333-IRAS4A proto-binary system using continuum emission and molecular tracers of shocked gas. Methods. We observed NGC 1333-IRAS4A in the SiO(6−5), SO(65−54), and CO(2−1) lines and the continuum emission at 1.3, 1.4, and 3 mm using the IRAM Plateau de Bure Interferometer in the framework of the CALYPSO large program. Results. We clearly disentangle for the first time the outflow emission from the two sources A1 and A2. The two protostellar jets have very different properties: the A1 jet is faster, has a short dynamical timescale (≲103 yr), and is associated with H2 shocked emission, whereas the A2 jet, which dominates the large-scale emission, is associated with diffuse emission, bends, and emits at slower velocities. The observed bending of the A2 jet is consistent with the change of propagation direction observed at large scale and suggests jet precession on very short timescales (~200−600 yr). In addition, a chemically rich spectrum with emission from several complex organic molecules (e.g. HCOOH, CH3OCHO, CH3OCH3) is only detected towards A2. Finally, very high-velocity shocked emission (~50 km s-1) is observed along the A1 jet. An LTE analysis shows that SiO, SO, and H2CO abundances in the gas phase are enhanced up to (3−4)×10-7, (1.4−1.7)×10-6, and (3−7.9)×10-7, respectively. Conclusions. The intrinsic different properties of the jets and driving sources in NGC 1333-IRAS4A suggest different evolutionary stages for the two protostars, with A1 being younger than A2, in a very early stage of star formation previous to the hot-corino phase.

Astrochemistry at work in the L1157-B1 shock: acetaldehyde formation

The formation of complex organic molecules (COMs) in protostellar environments is a hotly debated topic. In particular, the relative importance of the gas phase processes as compared to a direct formation of COMs on the dust grain surfaces is so far unknown. We report here the first high-resolution images of acetaldehyde (CH