Molecular Hydrogen Emission-line Objects Research Articles

Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Characterization of the young star T CrA and its circumstellar environment

Context. In recent years, a new hot topic has emerged in the star and planet formation field, namely, the interaction between the circumstellar disk and its birth cloud. The birth environments of young stars leave strong imprints on the star itself and their surroundings. In this context, we present a detailed analysis of the rich circumstellar environment around the young Herbig Ae/Be star T CrA. Aims. Our aim is to understand the nature of the stellar system and the extended circumstellar structures, as seen in scattered light images. Methods. We conducted our analysis on the basis of a set of combined archival data and new adaptive optics images at a high contrast and high resolution. Results. The scattered light images reveal the presence of a complex environment around T CrA, composed of a bright, forward-scattering rim of the disk's surface that is seen at very high inclinations, along with a dark lane of the disk midplane, bipolar outflows, and streamer features that are likely tracing infalling material from the surrounding birth cloud onto the disk. The analysis of the light curve suggests that the star is a binary with a period of 29.6 yr, confirming previous assertions based on spectro-astrometry. The comparison of the scattered light images with the ALMA continuum and 12CO (2–1) line emission shows that the disk is in Keplerian rotation and the northern side of the outflowing material is receding, while the southern side is approaching the observer. The overall system lies on different geometrical planes. The orbit of the binary star is perpendicular to the outflows and is seen edge on. The disk is itself seen edge-on, with a position angle of ~7°. The direction of the outflows seen in scattered light is in agreement with the direction of the more distant molecular hydrogen emission-line objects (MHOs) associated with the star. Modeling of the spectral energy distribution using a radiative transfer scheme is in good agreement with the proposed configuration, as well as the hydrodynamical simulation performed using a smoothed particle hydrodynamics code. Conclusions. We find evidence of streamers of accreting material around T CrA. These streamers connect the filament, along which T CrA is forming along with the outer parts of the disk, suggesting that the strong misalignment between the inner and outer disk is due to a change in the direction of the angular momentum of the material accreting on the disk during the late phase of star formation. This impacts the accretion taking place in the components of the binary, favoring the growth of the primary with respect the secondary, in contrast to the case of aligned disks.

Unveiling the substructure of the massive clump AGAL G035.1330−00.7450

Context. It is known that high-mass stars form as the result of the fragmentation of massive molecular clumps. However, it is not clear whether this fragmentation gives rise to cores that are massive enough to directly form high-mass stars, or if leads to cores of low and intermediate masses that generate high-mass stars by acquiring material from their environment. Aims. Detailed studies of massive clumps at the early stage of star formation are needed to collect observational evidence that sheds light on the fragmentation processes from clump to core scales. The infrared-quiet massive clump AGAL G035.1330−00.7450 (AGAL35) located at a distance of 2.1 kpc is a promising object for studying the fragmentation and the star formation activity at early stages. Methods. Using millimeter observations of continuum and molecular lines obtained from the Atacama Large Millimeter Array database at Bands 6 and 7, we studied the substructure of the source AGAL35. The angular resolution of the data at Band 7 is about 0.′′7, which allowed us to resolve structures of about 0.007 pc (~1500 au). Results. The continuum emission at Bands 6 and 7 shows that AGAL35 harbors four dust cores, labeled C1 to C4, with masses lower than 3 M⊙. Cores C3 and C4 exhibit well-collimated, young, and low-mass molecular outflows related to molecular hydrogen emission-line objects that were previously detected. Cores C1 and C2 present CH3CN J = 13–12 emission, from which we derive rotational temperatures of about 180 and 100 K, respectively. These temperatures allow us to estimate masses of about 1.4 and 0.9 M⊙ for C1 and C2, respectively, which are about an order of magnitude lower than those estimated in previous works and agree with the Jeans mass of this clump. In particular, the moment 1 map of CH3CN emission suggests the presence of a rotating disk towards C1, which is confirmed by the CH3OH and CH3OCHO (20–19) emissions. On the other hand, the CN N = 2–1 emission shows a clumpy and filamentary structure that seems to connect all the cores. These filaments might be tracing the remnant gas of the fragmentation processes taking place within the massive clump AGAL35 or the gas that is being transported toward the cores, which would imply a competitive accretion scenario. Conclusions. The massive clump AGAL35 harbors four low- to intermediate-mass cores with masses lower than 3 M⊙, which is about an order of magnitude smaller than the masses estimated in previous works. This study shows that in addition to the importance of high-resolution and sensitivity observations for a complete detection of all fragments, it is very important to accurately determine the temperature of these cores for a correct mass estimation. Finally, although no high-mass cores were detected toward AGAL35, the filamentary structure connecting all the cores means that high-mass stars might form through the competitive accretion mechanism.

MHOs toward HMOs: A Search for Molecular Hydrogen Emission-Line Objects toward High-mass Outflows

Abstract We present the results of a narrow-band near-infrared imaging survey for Molecular Hydrogen emission-line Objects (MHOs) toward 26 regions containing high-mass protostellar candidates and massive molecular outflows. We have detected a total of 236 MHOs, 156 of which are new detections, in 22 out of the 26 regions. We use H2 2.12 μm/H2 2.25 μm flux ratios, together with morphology, to separate the signatures of fluorescence associated with photo-dissociation regions (PDRs) from shocks associated with outflows in order to identify the MHOs. PDRs have typical low flux ratios of ∼1.5–3, while the vast majority of MHOs display flux ratios typical of C-type shocks (∼6–20). A few MHOs exhibit flux ratios consistent with expected values for J-type shocks (∼3–4), but these are located in regions that may be contaminated with fluorescent emission. Some previously reported MHOs have low flux ratios, and are likely parts of PDRs rather than shocks indicative of outflows. We identify a total of 36 outflows across the 22 target regions where MHOs were detected. In over half these regions, MHO arrangements and fluorescent structures trace features present in CO outflow maps, suggesting that the CO emission traces a combination of dynamical effects, which may include gas entrained in expanding PDRs as well as bipolar outflows. Where possible, we link MHO complexes to distinct outflows and identify candidate driving sources.

YSO jets in the Galactic plane from UWISH2 – III. Jets and outflows in Cassiopeia and Auriga

We present the analysis of 35.5 deg2 of images in the 1–0 S(1) line of H2 from the UK Widefield Infrared Survey for H2 (UWISH2) towards Cassiopeia and Auriga. We have identified 98 Molecular Hydrogen emission-line Objects (MHOs) driven by Young Stellar Objects, 60 per cent of which are bipolar outflows and all are new discoveries. We estimate that the UWISH2-extended emission object catalogue contains fewer than 2 per cent false positives and is complete at the 95 per cent level for jets and outflows brighter than the UWISH2 detection limit. We identified reliable driving source candidates for three quarters of the detected outflows, 40 per cent of which are associated with groups and clusters of stars. The driving source candidates are 20 per cent protostars, the remainder are Classical T-Tauri Stars. We also identified 15 new star cluster candidates near MHOs in the survey area. We find that the typical outflow identified in the sample has the following characteristics: the position angles are randomly orientated; bipolar outflows are straight within a few degrees; the two lobes are slightly asymmetrical in length and brightness; the length and brightness of the lobes are not correlated; typical time gaps between major ejections of material are 1–3 kyr, hence FU-Ori or EX-Ori eruptions are most likely not the cause of these, but we suggest MNors as a possible source. Furthermore, we find that outflow lobe length distributions are statistically different from the widely used total length distributions. There are a larger than expected number of bright outflows indicating that the flux distribution does not follow a power law.

A DEEP NEAR-INFRARED SURVEY TOWARD THE AQUILA MOLECULAR CLOUD. I. MOLECULAR HYDROGEN OUTFLOWS

We have performed an unbiased deep near-infrared survey toward the Aquila molecular cloud with a sky coverage of ~1 deg2. We identified 45 molecular hydrogen emission-line objects(MHOs), of which only 11 were previously known. Using the Spitzer archival data we also identified 802 young stellar objects (YSOs) in this region. Based on the morphology and the location of MHOs and YSO candidates, we associate 43 MHOs with 40 YSO candidates. The distribution of jet length shows an exponential decrease in the number of outflows with increasing length and the molecular hydrogen outflows seem to be oriented randomly. Moreover, there is no obvious correlation between jet lengths, jet opening angles, or jet H2 1-0 S(1) luminosities and spectral indices of the possible driving sources in this region. We also suggest that molecular hydrogen outflows in the Aquila molecular cloud are rather weak sources of turbulence, unlikely to generate the observed velocity dispersion in the region of survey.

Proper motions of molecular hydrogen outflows in theρOphiuchi molecular cloud

We aim to take a census of molecular hydrogen emission line objects (MHOs) in the Rho Ophiuchi molecular cloud and to make the first systematic proper motion measurements of these objects in this region. Deep H2 near-infrared imaging is performed to search for molecular hydrogen emission line objects. Multi-epoch data are used to derive the proper motions of the features of these objects, and the lengths and opening angles of the molecular hydrogen outflows. Our imaging covers an area of about 0.11 deg2 toward the L1688 core in the Rho Ophiuchi molecular cloud. In total, six new MHOs are discovered, 32 previously known MHOs are detected, and the proper motions for 86 features of the MHOs are measured. The proper motions lie in the range of 14 to 247 mas/yr, corresponding to transversal velocities of 8 to 140 km s-1 with a median velocity of about 35 km s-1. Based on morphology and proper motion measurements, 27 MHOs are ascribed to 21 driving sources. The molecular hydrogen outflows have a median length of about 0.04 pc and random orientations. We find no obvious correlation between H2 jet length, jet opening angle, and the evolutionary stage of the driving sources as defined by their spectral indices. We find that the fraction of protostars (23%) that drive molecular hydrogen outflows is similar to the one for Class II sources (15%). For most molecular hydrogen outflows, no obvious velocity variation along the outflow has been found. In Ophiuchus the frequency of occurrence of molecular hydrogen outflows has no strong dependency on the evolutionary stage of the driving source during the evolution from the protostellar stage to the classical T Tauri stage.

MASSIVE STAR FORMATION, OUTFLOWS, AND ANOMALOUS H2EMISSION IN Mol 121 (IRAS 20188+3928)

We have discovered 12 new molecular hydrogen emission-line objects (MHOs) in the vicinity of the candidate massive young stellar object Mol 121, in addition to five that were previously known. H2 2.12-micron/H2 2.25-micron flux ratios indicate another region dominated by fluorescence from a photo-dissociation region (PDR), and one region that displays an anomalously low H2 2.12-micron/H2 2.25-micron flux ratio (<1) and coincides with a previously reported deeply embedded source (DES). Continuum observations at 3 mm reveal five dense cores; the brightest core is coincident with the DES. The next brightest cores are both associated with cm continuum emission. One of these is coincident with the IRAS source; the other lies at the centroid of a compact outflow defined by bipolar MHOs. The brighter of these bipolar MHOs exhibits [Fe II] emission and both MHOs are associated with CH3OH maser emission observed at 95 GHz and 44 GHz. Masses and column densities of all five cores are consistent with theoretical predictions for massive star formation. Although it is impossible to associate all MHOs with driving sources in this region, it is evident that there are several outflows along different position angles, and some unambiguous associations can be made. We discuss implications of observed H2 2.12-micron/H2 2.25-micron and [Fe II] 1.64-micron/H2 2.12-micron flux ratios and compare the estimated total H2 luminosity with the bolometric luminosity of the region. We conclude that the outflows are driven by massive young stellar objects embedded in cores that are likely to be in different evolutionary stages.

YSO jets in the Galactic plane from UWISH2 - II. Outflow luminosity and length distributions in Serpens and Aquila

Jets and outflows accompany the mass accretion process in protostars and young stellar objects. Using a large and unbiased sample, they can be used to study statistically the local feedback they provide and the typical mass accretion history. Here we analyse such a sample of Molecular Hydrogen emission line Objects in the Serpens and Aquila part of the Galactic Plane. Distances are measured by foreground star counts with an accuracy of 25%. The resulting spacial distribution and outflow luminosities indicate that our objects sample the formation of intermediate mass objects. The outflows are unable to provide a sizeable fraction of energy and momentum to support, even locally, the turbulence levels in their surrounding molecular clouds. The fraction of parsec scale flows is one quarter and the typical dynamical jet age of the order of 1E4yrs. Groups of emission knots are ejected every 1E3yrs. This might indicate that low level accretion rate fluctuations and not FU-Ori type events are responsible for the episodic ejection of material. Better observational estimates of the FU-Ori duty cycle are needed.

Molecular hydrogen jets and outflows in the Serpens South filamentary cloud

We aimed to map the jets and outflows from the Serpens South star forming region and find an empirical relationship between the magnetic field and outflow orientation. Near-infrared H2 v=1-0 S(1) 2.122{\mu}m -line imaging of the \sim 30'-long filamentary shaped Serpens South star forming region was carried out. K s broadband imaging of the same region was used for continuum subraction. Candidate driving sources of the mapped jets/outflows are identified from the list of known protostars and young stars in this region, which was derived from studies using recent Spitzer and Herschel telescope observations. 14 Molecular Hydrogen emission-line objects(MHOs) are identified using our continuum-subtracted images. They are found to constitute ten individual flows. Out of these, nine flows are located in the lower-half(southern) part of the Serpens South filament, and one flow is located at the northern tip of the filament. Four flows are driven by well-identified Class 0 protostars, while the remaining six flows are driven by candidate protostars mostly in the Class I stage, based on the Spitzer and Herschel observations. The orientation of the outflows is systematically perpendicular to the direction of the near-infrared polarization vector, recently published in the literature. No significant correlation was observed between the orientation of the flows and the axis of the filamentary cloud.

Jet-driving protostars identified from infrared observations of the Carina Nebula complex

Aims: Jets are excellent signposts for very young embedded protostars, so we want to identify jet-driving protostars as a tracer of the currently forming generation of stars in the Carina Nebula, which is one of the most massive galactic star-forming regions and which is characterised by particularly high levels of massive-star feedback on the surrounding clouds. Methods: We used archive data to construct large (> 2 deg x 2 deg) Spitzer IRAC mosaics of the Carina Nebula and performed a spatially complete search for objects with excesses in the 4.5 micron band, typical of shock-excited molecular hydrogen emission. We also identified the mid-infrared point sources that are the likely drivers of previously discovered Herbig-Haro jets and molecular hydrogen emission line objects. We combined the Spitzer photometry with our recent Herschel far-infrared data to construct the spectral energy distributions, and used the Robitaille radiative-transfer modelling tool to infer the properties of the objects. Results: The radiative-transfer modelling suggests that the jet sources are protostars with masses between ~1 M_sol and ~10 M_sol that are surrounded by circumstellar disks and embedded in circumstellar envelopes. Conclusions: The estimated protostar masses < 10 M_sol suggest that the current star-formation activity in the Carina Nebula is restricted to low- and intermediate-mass stars. More optical than infrared jets can be observed, indicating that star formation predominantly takes place close to the surfaces of clouds.

H2flows in the Corona Australis cloud and their driving sources

We uncover the H2 flows in the Corona Australis molecular cloud and in particular identify the flows from the Coronet cluster. Near-infrared H2 v=1--0 S(1), 2.12micron-line, narrow-band imaging survey of the R CrA cloud core was carried out. We identify the best candidate-driving source for each outflow by comparing the flow properties, available proper motions, and the known/estimated properties of the driving sources. We also adopted the thumbrule of outflow power as proportional to source luminosity and inversely proportional to the source age to reach a consensus. Results: Continuum-subtracted, narrow-band images reveal several new Molecular Hydrogen emission-line Objects (MHOs). Together with previously known MHOs and Herbig-Haro objects we catalog at least 14 individual flow components of which 11 appear to be driven by the RCrA aggregate members. The flows originating in the Coronet cluster have lengths of ~0.1-0.2 pc. Eight out of nine submillimeter cores mapped in the Coronet cluster region display embedded stars driving an outflow component. Roughly 80% of the youngest objects in the Coronet are associated with outflows. The MHO flows to the west of the Coronet display lobes moving to the west and vice-versa, resulting in nondetections of the counter lobe in our deep imaging. We speculate that these counterflows may be experiencing a stunting effect in penetrating the dense central core. Conclusions:Although this work has reduced the ambiguities for many flows in the Coronet region, one of the brightest H2 feature (MHO2014) and a few fainter features in the region remain unassociated with a clear driving source. The flows from Coronet, therefore, continue to be interesting targets for future studies.

A general catalogue of molecular hydrogen emission-line objects (MHOs) in outflows from young stars

We present a catalogue of Molecular Hydrogen emission-line Objects (MHOs) in outflows from young stars, most of which are embedded. All objects are identified in the near-infrared lines of molecular hydrogen, all reside in the Milky Way, and all are associated with jets or molecular outflows from young stars. Objects in both low and high-mass star forming regions are included. This catalogue complements the existing database of Herbig-Haro objects; indeed, for completeness, HH objects that are detected in H2 emission are included in the MHO catalogue.