T. Giannini

Far-Infrared Investigation of Class 0 Sources: Line Cooling*

We have investigated with the Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO) the far-infrared spectra (43-197 μm) of a sample of 17 class 0 sources and their associated outflows. In addition to [O I] 63 μm, the pure rotational lines of abundant molecules such as CO, H2O, and OH are frequently observed in these sources, at variance with more evolved young stellar objects. We found, in agreement with previous studies conducted on individual sources, that the molecular line excitation arises from small regions, with typical sizes of 10-9 sr, of warm (200 < T < 2000 K) and dense gas (104 < n < 107 cm-3), compressed after the passage of shocks. In particular, we found slow, nondissociative shocks as the main mechanism at the origin of the molecular gas heating, while the bulk of the [O I] 63 μm line emission is due to the dissociative J-shock component arising from the Mach disk at the head of the protostellar jet, as testified by the fact that this line emission happens to be a good tracer of the source mass-loss rate. Large abundances of gas-phase H2O are commonly estimated, with values that appear to be correlated with the gas temperature. The total far-infrared (FIR) line cooling LFIR = L(O ) + L(CO) + L(H2O) + L(OH), which amounts to ~10-2 to 10-1 L☉, is roughly equal to the outflow kinetic luminosity as estimated by means of millimeter molecular mapping. This circumstance demonstrates that the FIR line cooling can be a valid direct measure of the power deposited in the outflow, not affected by geometrical or opacity problems like the determination of Lkin or by extinction problems like the near-infrared shocked H2 emission. We finally remark that the strong molecular emission observed, and in particular H2O emission, is a peculiarity of the environments of class 0 sources. The present analysis shows that the ratio between FIR molecular line luminosity and bolometric luminosity (Lmol/Lbol) is always larger than ~10-3 in class 0 objects. We suggest that this parameter could be used as a further criterion for identifying future class 0 candidates.

Recipes for stellar jets: results of combined optical/infrared diagnostics

We examine the conditions of the plasma along a sample of “classical” Herbig-Haro (HH) jets located in the Orion and Vela star forming regions, through combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH 34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed quasi-simultaneously and in the same manner at moderate spatial/spectral resolution. Once intercalibrated, the obtained spectra cover a wide wavelength range from 0.6−2.5 µm, including many transitions from regions of different excitation conditions. This allows us to probe the density and temperature stratification which characterises the cooling zones behind the shock fronts along the jet. From the line ratios we derive the variation of the visual extinction along the flow, the electron density and temperature (ne and Te), the hydrogen ionisation fraction xe, and the total density nH in the emission region of different lines. The knowledge of such parameters is essential for testing existing jet models and for planning follow-up high-angular resolution observations. From the diagnostics of optical forbidden lines we find, on average, that in the examined jets, in the region of optical emission, ne varies between 50 cm −3 and 3 × 10 3 cm −3 , xe ranges between 0.03 and 0.6, and the electron temperature Te is ∼1.3 × 10 4 Ki n the HH 111 and HH 34 jets, while it appears to be higher (1.8 × 10 4 K on average) in the other examined jets. The electron density and temperature derived from [Fe ii] lines, turn out to be, respectively, higher and lower in comparison to those determined from optical lines, in agreement with the fact that the [Fe ii] lines arise in the more compressed gas located further from the shock front. An even denser component in the jets, with values of ne up to 10 6 cm −3 is detected using the ratio of calcium lines. The derived physical parameters are used to estimate the depletion onto dust grains of calcium and iron with respect to solar abundances. This turns out to be quite substantial, being between 70% and 0% for Ca and ∼90% for Fe. This leads us to suggest that the weak shocks present in the beams are not capable of completely destroying the ambient dust grains, confirming previous theoretical studies. We then derive the mass flux rates, u Mjet, in the flows using two independent methods. Taking into account the filling factor of the emitting gas, u –– –– –– –

Water cooling of shocks in protostellar outflows: Herschel-PACS map of L1157

Context. The far-IR/sub-mm spectral mapping facility provided by the Herschel-PACS and HIFI instruments has made it possible to obtain, for the first time, images of H2O emission with a spatial resolution comparable to ground based mm/sub-mm observations. Aims. In the framework of the Water In Star-forming regions with Herschel (WISH) key program, maps in water lines of several outflows from young stars are being obtained, to study the water production in shocks and its role in the outflow cooling. This paper reports the first results of this program, presenting a PACS map of the o-H2O 179 mu m transition obtained toward the young outflow L1157. Methods. The 179 mu m map is compared with those of other important shock tracers, and with previous single-pointing ISO, SWAS, and Odin water observations of the same source that allow us to constrain the H2O abundance and total cooling. Results. Strong H2O peaks are localized on both shocked emission knots and the central source position. The H2O 179 mu m emission is spatially correlated with emission from H-2 rotational lines, excited in shocks leading to a significant enhancement of the water abundance. Water emission peaks along the outflow also correlate with peaks of other shock-produced molecular species, such as SiO and NH3. A strong H2O peak is also observed at the location of the proto-star, where none of the other molecules have significant emission. The absolute 179 mu m intensity and its intensity ratio to the H2O 557 GHz line previously observed with Odin/SWAS indicate that the water emission originates in warm compact clumps, spatially unresolved by PACS, having a H2O abundance of the order of 10(-4). This testifies that the clumps have been heated for a time long enough to allow the conversion of almost all the available gas-phase oxygen into water. The total H2O cooling is similar to 10(-1) L-circle dot, about 40% of the cooling due to H-2 and 23% of the total energy released in shocks along the L1157 outflow.

1–2.5

As part of a 1-2.5m spectroscopic survey of jets and molecular outflows, we present the spectra of three Herbig Haro chains (HH111, HH240/241, HH120) characterized by strong emission from several Feii transitions originating from the first 13 fine structure levels. Such emission is correlated with optical Sii emission and appears to decrease moving away from the driving source. From the analysis of the Feii lines we have derived electron densities values in the range 3 10 3 - 2 10 4 cm 3 , which are systematically larger than those inferred from optical Sii line ratios. We suggest that Feii lines, having critical densities higher than the optical Sii transitions, trace either regions of the post-shock cooling layers with higher compression, or a section of the jet axis at a higher degree of ionization. Strong H2 emission lines are also detected along the three flows and their analysis indicates that a combination of dierent shocks can be responsible for their excitation in the dierent objects. Consequently the Feii line emission, which requires the presence of fast dissociative shocks, is completely independent from the excitation mechanism giving rise to the molecular emission. In addition to the Feii and H2 lines, emission from other species such as Ci ,S ii ,N i as well as recombination lines from the Paschen series are detected and have been used as a reference to infer the gas-phase iron abundance in the observed HH objects. We estimate a grain destruction eciency of about 30-60%: the highest value is found for HH240A, which also shows the highest degree of excitation among the observed objects.

\mu

We report the results of spectroscopic mapping observations carried out toward protostellar outflows in the BHR71, L1157, L1448, NGC 2071, and VLA 1623 molecular regions using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations, covering the 5.2-37 μm spectral region, provide detailed maps of the eight lowest pure rotational lines of molecular hydrogen and of the [S I] 25.25 μm and [Fe II] 26.0 μm fine-structure lines. The molecular hydrogen lines, believed to account for a large fraction of the radiative cooling from warm molecular gas that has been heated by a non-dissociative shock, allow the energetics of the outflows to be elucidated. Within the regions mapped toward these five outflow sources, total H2 luminosities ranging from 0.02 to 0.75 L ☉ were inferred for the sum of the eight lowest pure rotational transitions. By contrast, the much weaker [Fe II] 26.0 μm fine-structure transition traces faster, dissociative shocks; here, only a small fraction of the fast shock luminosity emerges as line radiation that can be detected with Spitzer/IRS.

m spectra of jets from young stars: Strong

Complete flux-calibrated spectra covering the spectral range from 6000u to 2.5µm have been obtained along the HH1 jet and analysed in order to explore the potential of a combined optical/near-IR diagnostic applied to jets from young stellar objects. The main physical parameters (visual extinction, electron temperature and density, ionization fraction and total density) have been derived along the jet using various diagnostic line ratios. This multi-line analysis shows, in each spatially unresolved knot, the presence of zones at different excitation conditions, as expected from the cooling layers behind a shock front. In particular, a density stratification in the jet is evident from ratios of various lines of different critical density. We measure electron densities in the range 610 2 -310 3 cm −3 with the (S ii) optical doublet lines, 410 3 -10 4 cm −3 with the near-IR (Fe ii) lines, and 10 5 -10 6 cm −3 with optical (Fe ii) and CaII lines. The electron temperature also shows variations, with values between 8000-11000 K derived from optical/near-IR (Fe ii) lines and 11000-20000 K from a combined diagnostic employing optical (O i) and (N ii) lines. Thus (Fe ii) lines originate in a cooling layer located at larger distances from the shock front than that generating the optical lines, where the compression is higher and the temperature is declining. The derived parameters were used to measure the mass flux along the jet, adopting different procedures, the advantages and limitations of which are discussed. The (Fe ii)1.64µm line luminosity turns out to be more suitable to measure u Mjet than the optical lines, since it samples a fraction of the total mass flowing through a knot larger than the (O i) or (S ii) lines. u Mjet is high in the initial part of the flow (�2.210 −7 M⊙ yr −1 ) but decreases by about an order of magnitude further out. Conversely, the mass flux associated with the warm molecular material is low, u MH2�10 −9 M⊙ yr −1 , and does not show appreciable variations along the jet. We suggest that part of the mass flux in the external regions is not revealed in optical and IR lines because it is associated with a colder atomic component, which may be traced by the far-IR (O i)63µm line. Finally, we find that the gas-phase abundance of refractory species, such as Fe, C, Ca, and Ni, is lower than the solar value, with the lowest values (between 10 and 30% of solar) derived in the inner and densest regions. This suggests a significant fraction of dust grains may still be present in the jet beam, imposing constraints on the efficiency of grain destruction by multiple low-velocity shock events.

\ion{Fe}{ii}

We performed a sensitive search for the ground-state emission lines of ortho- and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the 1_{11}--0_{00} line. We report a very tentative detection, however, of the 1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s. The latter constitutes a 6sigma detection. Regardless of the reality of this tentative detection, model calculations indicate that our sensitive limits on the line strengths preclude efficient desorption of water in the UV illuminated regions of the disk. We hypothesize that more than 95-99% of the water ice is locked up in coagulated grains that have settled to the midplane.

emission in HH111, HH240-241 and HH120

Aims. We present the results of a spectroscopic analysis of three young embedded sources (HH26 IRS, HH34 IRS, and HH46 IRS) belonging to different star-forming regions and displaying well-developed jet structures. The aim is to investigate the source accretion and ejection properties and their connection. Methods. We used VLT-ISAAC near-IR medium resolution (R ∼ 9000) spectra (H and K bands) to derive, in a self-consistent way, parameters like the star luminosity, accretion luminosity, and the mass accretion rate. Mass ejection rates have also been estimated from analysing different emission features. Results. The spectra present several emission lines but no photospheric features in absorption, indicating a large veiling in both the H and K bands. In addition to features commonly observed in jet driving sources ([Fe ii], H2 ,H i, CO), we detect a number of emission lines due to permitted atomic transitions, such as Na i and Ti i that are only 2−5 times weaker than the Brγ line. Some of these features remain unidentified. Emission from Na i 2.2 µm doublet is observed along with CO(2−0) band-head emission, indicating a common origin in an inner gaseous disc heated by accretion. We find that accretion provides about 50% and 80% of the bolometric luminosity

Spitzer spectral line mapping of protostellar outflows. i. basic data and outflow energetics

The far-infrared spectra (45-197 μm) of 28 low-luminosity young embedded objects have been studied in order to search for possible evolutive trends in the observed spectral features. The low-resolution spectra from 45 to 197 μm of 17 Class 0 and 11 Class I sources taken with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory have been used for this analysis. The most prominent features presented by these spectra are the [O I] 63 and 145 μm fine-structure lines and pure rotational lines from the abundant molecules CO, H2O, and OH. Clear differences are found, however, between the spectra of the two classes of objects. Water lines, which are prominent in the spectra of Class 0 sources, are not observed in Class I objects, with an upper limit 10-5 on the H2O abundance. Furthermore, the total cooling due to molecular emission in Class 0 sources is on average significantly larger than in Class I sources, while the cooling due to atomic oxygen is fairly constant among the two classes of objects. Finally, the total gas cooling as traced by the far-infrared lines (LFIR) is correlated with the bolometric luminosity for the Class 0 sample of sources, with an LFIR/Lbol ratio (~10-2) of about an order of magnitude larger than in Class I sources. We suggest that most of the observed emission lines originate from shocks at the base and along the source outflows. In such a case these results can be interpreted in terms of a change in the modality of the interaction between the protostellar jet and the circumstellar environment. During the Class 0 phase the impact of energetic flows with the dense ambient medium gives rise to a strong component of nondissociative C-type shock, while during the Class I phase such impact produces less energetic shocks with an enhanced dissociative J-type component. Finally, the low H2O abundance found in Class I sources can be explained by the action of the progressively less shielded interstellar UV field.

A combined optical/infrared spectral diagnostic analysis of the HH1 jet

Near IR spectra at low (R ∼ 800) and medium (R ∼ 9000) resolution, obtained with ISAAC at VLT, have been used to pose constraints on the evolutionary state and accretion properties of a sample of five embedded YSOs located in the R CrA core. This sample includes three Class I sources (HH100 IR, IRS2 and IRS5), and two sources with NIR excesses (IRS6 and IRS3). IRS5 and IRS6 have been discovered to be binaries with a separation between the two components of 78 and 97 AU, respectively. Absorption lines, typical of late-type photospheres, have been detected in the medium resolution spectra of all the observed targets, including HH 100 IR and IRS2 which have high values of infrared continuum veiling (r K = 6 and 3, respectively). These two sources also present low resolution spectra rich in emission lines (HI, CO and plenty of other permitted lines from neutral atoms) likely originating in the disk-star-wind connected regions. Among the features observed in HH100 IR and IRS2, Na I at 2.205 μm and CO at 2.3 μm, which are more commonly used for stellar classification, are detected in emission instead of absorption. Several strong photospheric lines which lie around 2.12 and 2.23 μm and whose ratio is sensitive to both effective temperature and gravity are proposed as independent diagnostic tools for this type of sources. We derived spectral types, veiling and stellar luminosity for the five observed sources, which in turn have been used to infer their mass (ranging between 0.3-1.2 M ○. ) and age (between 10 5 and 10 6 yr) adopting pre-main sequence evolutionary tracks. We find that in HH100 IR and IRS2 most of the bolometric luminosity is due to accretion (L acc /L bol ∼ 0.8 and 0.6 respectively), while the other three investigated sources, including the Class I object IRS5a, present low accretion activity (L acc /L bol < 0.2). Mass accretion rates of the order of 2 x 10 -6 and 3 x 10 -7 M ○. yr -1 are derived for HH100 IR and IRS2, respectively, i.e. higher by an order of magnitude than the average values derived for T Tauri stars. We observe a general correlation between the accretion luminosity, the IR veiling and the emission line activity of the sources. In particular, we find that the correlation between L acc and L Brγ , previously reported for optical T Tauri stars, can be extended to the embedded sources, up to at least one order of magnitude larger line luminosity. A correlation between the accretion activity and the spectral energy distribution slope is recognizable with the notable exception of IRS5a. Our analysis therefore shows how the definition of the evolutionary stage of deeply embedded YSOs by means of IR colors needs to be refined.