D. Lorenzetti

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.

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

IR observations in J, H, K, L, M, and 8-13 μm bands of Herbig Ae/Be stars located in the southern hemisphere are presented. These results enlarge an already existing data base, increasing the significance of the correlations which can be obtained from observational parameters. Silicate features detected both in emission and in absorption indicate the presence of dust around these objects. A first analysis based on two-color diagrams, polarization, and luminosity suggests that the spherical geometry for the dust distribution is a more common feature, with respect to the flattened structure

m spectra of jets from young stars: Strong

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

\ion{Fe}{ii}

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.

emission in HH111, HH240-241 and HH120

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.

Infrared emission from dust structures surrounding Herbig Ae/Be stars

MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for the Very Large Telescope (VLT), selected by ESO for a Phase A study. The baseline design consists of ~1000 fibers deployable over a field of view of ~500 square arcmin, the largest patrol field offered by the Nasmyth focus at the VLT. The total wavelength coverage is 0.8μm-1.8μm and two resolution modes: medium resolution and high resolution. In the medium resolution mode (R~4,000-6,000) the entire wavelength range 0.8μm-1.8μm is observed simultaneously, while the high resolution mode covers simultaneously three selected spectral regions: one around the CaII triplet (at R~8,000) to measure radial velocities, and two regions at R~20,000 one in the J-band and one in the H-band, for detailed measurements of chemical abundances. The grasp of the 8.2m Very Large Telescope (VLT) combined with the large multiplex and wavelength coverage of MOONS – extending into the near-IR – will provide the observational power necessary to study galaxy formation and evolution over the entire history of the Universe, from our Milky Way, through the redshift desert and up to the epoch of re-ionization at z<8-9. At the same time, the high spectral resolution mode will allow astronomers to study chemical abundances of stars in our Galaxy, in particular in the highly obscured regions of the Bulge, and provide the necessary follow-up of the Gaia mission. Such characteristics and versatility make MOONS the long-awaited workhorse near-IR MOS for the VLT, which will perfectly complement optical spectroscopy performed by FLAMES and VIMOS.

Accretion and ejection properties of embedded protostars: the case of HH26, HH34, and HH46 IRS

Observations of the prompt afterglow of Gamma Ray Burst (GRB) events are unanimously considered of paramount importance for GRB science and related cosmology. Such observations at NIR wavelengths are even more promis- ing allowing one to monitor high-z Ly- absorbed bursts as well as events occurring in dusty star-forming regions. In these pages we present REM (Rapid Eye Mount), a fully robotized fast slewing telescope equipped with a high throughput NIR (Z, J, H, K) camera dedicated to detecting the prompt IR afterglow. REM can discover objects at extremely high redshift and trigger large telescopes to observe them. The REM telescope will simultaneously feed ROSS (REM Optical Slitless Spectrograph) via a dichroic. ROSS will intensively monitor the prompt optical continuum of GRB afterglows. The synergy between REM-IR cam and ROSS makes REM a powerful observing tool for any kind of fast transient phenomena.

Evolution in the Far-Infrared Spectra of Low-Mass Young Embedded Sources

The spectral energy distributions (SEDs) observed in Herbig Ae/Be stars are critically analyzed in connection with a spherically symmetric model for matter distribution around the central objects. The possibility that the dusty component is described by an MRN-DL mixture (see the work of Mathis, Rumpl, and Nordsiek; Draine and Lee) and is constituted by particles with mean sizes greater than those typical of the interstellar medium is considered. We find that while spherical models with interstellar-like dust are able to describe the observed SEDs in the visible, infrared, and radio region, they generally fail to fit the submillimeter fluxes. A better agreement is obtained if the absorption coefficient of the dust at far-infrared wavelengths follows a dependence that is shallower than that of the standard MRN-DL mixture. In this case, general agreement among the observed and the model-derived spectral types, distance, and visual extinction is found. Inconsistencies in submillimeter fluxes obtained by different authors are discussed in connection with the different telescopes used and the confusion of the sources. An indication is given for the density distribution around Herbig Ae/Be stars, which, if modeled with a power law, goes as the inverse of the radial distance in most cases.

Probing the embedded YSOs of the R CrA region through VLT-ISAAC spectroscopy ,

We present low-resolution spectroscopy in the near-IR (0.8-2.5 μm) of the EXor variables. These are the initial results (obtained during the period 2007-2008) from a long-term photometric and spectroscopic program aimed to study the variability in the accretion processes of pre-main-sequence stars, by correlating the continuum fluctuations with the spectroscopic properties. Eight sources have been observed in different epochs, for a total of 25 acquired spectra. EXor spectra show a wide variety of emission features dominated by H I recombination (Paschen and Brackett series). We have investigated whether line and continuum variability could be due to a variable extinction, but such a hypothesis is applicable only to the peculiar source PV Cep. By comparing the observed spectra with a wind model, mass loss rates in the range (2-10) × 10–8 M ☉ yr–1 are derived, along with other wind parameters. Consistent results are also obtained by assuming that H I lines are due to accretion. A CO overtone is also detected in the majority of the sources both in absorption and in emission. It appears to come from regions more compact than winds, likely the stellar photosphere (when in absorption) and the circumstellar disk (when in emission). Na I and Ca I IR lines behave as the CO does, thus they are thought to arise in the same locations. For some targets multiple spectra correspond to different activity stages of the source. Those exhibiting the largest continuum variation at 2 μm (ΔK 1 mag) present a significant line flux fading during the continuum declining phases. In particular, CO absorption (emission) appears associated with inactive (active) stages, respectively.