Low-mass YSOs Research Articles

The RMS survey

Context. The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, high-resolution mid-infrared colour-selected sample of massive young stellar objects (MYSOs). We have identified ∼2000 MYSO candidates located within our Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of our follow-up observations is to identify other objects with similar colours such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low-mass YSOs. Aims. A critical part of our follow-up programme is to conduct 13CO molecular line observations in order to determine kinematic distances to all of our MYSO candidates. These distances will be used in combination with far-IR and (sub)millimetre fluxes to determine bolometric luminosities which will allow us to identify and remove nearby low-mass YSOs. In addition these molecular line observations will help in identifying evolved stars which are weak CO emitters. Methods. We have used the 15 m James Clerk Maxwell Telescope (JCMT), the 13.7 m telescope of the Purple Mountain Observatory (PMO), the 20 m Onsala telescope and the 22 m Mopra telescope to conduct molecular line observations towards 508 MYSOs candidates located in the 1st and 2nd Quadrants. These observations have been made at the J = 1−0 (Mopra, Onsala and PMO) and J = 2−1 (JCMT) rotational transition frequency of 13CO molecules and have a spatial resolution of ∼20′′−55′′, a sensitivity of T ∗ A 0.1 K and a velocity resolution of ∼0.2 km s−1. We complement these targeted observations with 13CO spectra extracted from the Galactic Ring Survey (GRS), which have a velocity resolution of ∼0.21 km s−1 and sensitivity T ∗ A 0.13−0.2 K, towards a further 403 RMS sources. Results. In this paper we present the results and analysis of the 13CO spectra obtained towards 911 MYSO candidates. We detect 13CO emission towards 780 RMS sources which corresponds to approximately 84% of those observed. A total of 2595 emission components are detected above 3σ level (typically T ∗ A ≥ 0.3 K), with multiple components being observed towards the majority of these sources – 520 sources (∼56%) – with an average of ∼4 molecular clouds detected along each line of sight. These multiple emission features make it difficult to assign a unique kinematic velocity to many of our sample. We have used archival CS (J = 2−1) and maser velocities to resolve the component multiplicity towards 175 sources (∼20%) and have derived a criterion which is used to identify the most likely component for a further 191 multiple component sources. Combined with the single component detections we have obtained unambiguous kinematic velocities for 638 of the 780 MYSOs candidates towards which CO is detected (∼80% of the detections). The 141 sources for which we have not been able to determine the kinematic velocity will require additional line data. Using the rotation curve of Brand and Blitz (1993) and their radial velocities we calculate kinematic distances for all detected components.

Disc signatures in a new population of low mass YSOs in ρ Ophiuchi

AbstractThe deepest near-IR variability survey of the ρ Ophiuchi cluster with the WFCAM/UKIRT has been used to uncover a new population of young low mass objects. Evidence for the existence of discs around the new objects has been found when combining data with IRAC/Spitzer observations. A new insight on the frequency and dynamics of discs around low mass YSOs is given, essential for understanding the origin and evolution of circumstellar discs and ultimately the environment for planet formation.

Footprints of triggering in large area surveys of the nearby ISM and YSOs

AbstractOur goal is to evaluate the role of triggering effects on the star formation and early stellar evolution by presenting a statistically large sample of cloud and low-mass YSO data. We conducted large area surveys (ranging from 400 square-degree to 10800 square-degree) in optical, NIR and FIR. The distribution of the ISM and low-mass YSOs were surveyed. A relative excess was found statistically in the number of dense and cold core bearing clouds and low mass YSOs in the direction of the FIR loop shells indicating a possible excess in their formation.

Dynamical processes in star forming regions: feedback and turbulence generation

AbstractYoung stellar objects (YSOs) inject large amounts of momentum and kinetic energy into their surroundings. Feedback from low mass YSOs is dominated by their outflows. However, as stellar mass increases, UV photo-heating and ionization play increasingly important roles. Massive stars produce powerful stellar winds and explode as supernovae within 3 – 40 Myr after birth. While low-mass protostellar feedback can drive turbulence in cloud cores and even disrupt the star forming environment, feedback from massive stars plays important roles in the generation of cloud structure and motions in the entire ISM.

The Massive Bipolar Outflow in IRAS 20110+3321

Mapping observations were made towards IRAS 20110+3321 using theNobeyama 45 m and the Delingha 13.7 m radio telescopes. The highangular resolution (~21′′) image with the 45 m telescope showsthat there is a high-velocity bipolar molecular outflow in this region,which is in the NW–SE direction with a collimation factor of ~ 2.2.The outflow has significantly higher mass loss rate and mechanicalluminosity than those from low mass YSOs, indicating that the outflow isdriven by the IRAS source. A dense massive core was detected by mappingC18O (J = 1−0) line in the area with the 13.7 m telescope. TheIRAS source lies within the core but slightly offsets from its emissionpeak.

First results from a VLBA proper motion survey of H$_\mathsf{2}$O masers in low-mass YSOs: the Serpens core and RNO 15-FIR

This article reports first results of a long-term observational program aimed to study the earliest evolution of jet/disk systems in low-mass YSOs by means of VLBI observations of the 22.2 GHz water masers. We report here data for the cluster of low-mass YSOs in the Serpens molecular core and for the single object RNO~15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the Serpens cluster, the water maser emission comes from two small (< 5 AU in size) clusters of features separated by ~25 AU, having line of sight velocities strongly red-shifted (by more than 10 km/s) with respect to the LSR velocity of the molecular cloud. The two maser clusters are oriented on the sky along a direction that is approximately perpendicular to the axis of the radio continuum jet observed with the VLA towards SMM1. The spatial and velocity distribution of the maser features lead us to favor the interpretation that the maser emission is excited by interaction of the receding lobe of the jet with dense gas in the accretion disk surrounding the YSO in SMM1. Towards RNO~15-FIR, the few detected maser features have both positions and (absolute) velocities aligned along a direction that is parallel to the axis of the molecular outflow observed on much larger angular scales. In this case the maser emission likely emerges from dense, shocked molecular clumps displaced along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1 is more massive than the one in RNO~15-FIR. We discuss the case where a high mass ejection rate can generate jets sufficiently powerful to sweep away from their course the densest portions of circumstellar gas. In this case, the excitation conditions for water masers might preferably occur at the interface between the jet and the accretion disk, rather than along the jet axis.

A 3–5 $\mathsf{\mu}$m VLT spectroscopic survey of embedded young low mass stars II

The 4.62 µm (2164.5 cm −1 ) band has been detected in the M-band spectra of 34 deeply embedded young stellar objects (YSO's), observed with high signal-to-noise and high spectral resolution with the VLT-ISAAC spectrometer, providing the first opportunity to study the solid OCN − abundance toward a large number of low-mass YSO's. It is shown unequivocally that at least two components, centred at 2165.7 cm −1 (FWHM = 26 cm −1 ) and 2175.4 cm −1 (FWHM = 15 cm −1 ), underlie the XCN band. Only the 2165.7-component can be ascribed to OCN − , embedded in a strongly hydrogen-bonding, and possibly thermally annealed, ice environment based on laboratory OCN − spectra. In order to correct for the contribution of the 2175.4-component to the XCN band, a phenomenological decomposition into the 2165.7- and the 2175.4-components is used to fit the full band profile and derive the OCN − abundance for each line-of-sight. The same analysis is performed for 5 high-mass YSO's taken from the ISO-SWS data archive. Inferred OCN − abundances are ≤0.85% toward low-mass YSO's and ≤1% toward high-mass YSO's, except for W33 A. Abundances are found to vary by at least a factor of 10-20 and large source-to-source abundance variations are observed within the same star-forming cloud complex on scales down to 400 AU, indicating that the OCN − formation mechanism is sensitive to local conditions. The inferred abundances allow quantitatively for photochemical formation of OCN − , but the large abundance variations are not easily explained in this scenario unless local radiation sources or special geometries are invoked. Surface chemistry should therefore be considered as an alternative formation mechanism.

Star-forming protoclusters associated with methanol masers

We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong ($>100$ mJy) radio continuum source: G 31.28+0.06, G 59.78+0.06, G 173.49+2.42 (S231, S233IR), G 188.95+0.89 (S252, AFGL5180) and G 192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultra-compact \ion{H}{ii} regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of $8''-34''$. Spectral energy distribution (SED) diagrams for each site are presented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive ($>50$ M$_{\odot}$), deeply embedded ($A_v>40$ mag) and very luminous ($>10^4$ S L$_{\odot}$) molecular clump, with $L_{total}{\propto}M_{gas}^{0.75}$. These physical properties characterise massive star-forming clumps in earlier evolutionary phases than \ion{H}{ii} regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colderclumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wavelengths, to an (ultra-compact) \ion{H}{ii} region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index ($\beta$) range between 1.6 and 1.9.

Collimated molecular jets from high-mass young stars: IRAS 18151-1208

Recent near-IR images of massive star forming regions have revealed two collimated jets in the IRAS 18151-1208 region, one of which is almost a parsec in length (Varricatt et al.). Follow-up high-spectral-resolution echelle spectroscopy and 2-dimensional “integral field” spectroscopy of the associated molecular shock features are presented here. From these data kinematic information and excitation maps are extracted, which show that the two jets are morphologically, kinematically and energetically similar to their counterparts from low mass protostars. The close association between the H2 emission features and the high-velocity CO emission presented by Beuther et al. also suggests that the CO represents gas entrained by these two very collimated jets. From the mass and momentum of the molecular gas, and the luminosity of the H2 features, it is clear that the flows must be powered by massive sources. To all intents and purposes, the molecular jets appear to be scaled-up versions of low-mass YSO jets. Collectively, the observations add further support to the idea that massive stars are formed through vigorous disk accretion, and that, while in their earliest stages of evolution, massive protostars drive collimated jets.

A search for shock-excited molecular hydrogen knots in Chamaeleon I very low mass YSOs

We have obtained narrow-band images of three selected areas of the Chamaeleon I dark cloud which harbor very low mass young stars, centered on the H 2 and Br γ lines and neighboring continuum as well as on the broad band K s . One region is located in the northern part of the cloud, roughly coinciding with the densest area. The other two regions are in the southern section of the cloud. Our aim is to search for H 2 outflows associated with these objects. In the northern region, we found seven new H 2 knots, five of which are aligned in the direction of a previously known 12 CO molecular bipolar outflow. Further evidence that the class I low mass stellar object ISO-ChaI 192 is the driving source of the molecular flow is given by the presence of a 960 AU long elongated structure at 2.2 μ m emanating from this star and oriented parallel to the bipolar structure. Another pair of H 2 knots, although lying relatively nearby, is not aligned with the outflow direction. They are located on opposite sides of C1–6, a low mass class II object in the northern part of the Chamaeleon I dark cloud. In contrast, we fail to detect any H 2 emission object brighter than our sensitivity limit (~$6 \times 10^{-32}$ W/m 2 Hz arcsec 2 ) in the two southern areas of the cloud that also harbor several very low mass stars, including two transition stellar/sub-stellar objects. This negative result is probably not surprising in view of the extremely low accretion rates measured for brown dwarfs ($\dot{M} \sim 10^{-12} {-}10^{-9} ~ M_{\odot}$ yr -1 ). Deeper H 2 observations are required to better constraint the outflow event in sub-stellar objects.

Very cold cores in the Taurus Molecular Ring as seen by ISO

Three prominent cold objects of the Taurus Molecular Ring (TMR) were revealed by our ISOPHOT 200 µ mm ap of the south-eastern part of the Heiles Cloud 2 (HCL 2) cloud complex. One corresponds to the cyanopolyyne peak region of the TMC-1 ridge, (TMC-1 CP), one is the HCL2-E cloud, and one which we call HCL2-ES lies south of TMC-1. The 200/100 µm colour temperatures and column densities of the three ISOPHOT cold clouds are ≈12 K, and 1.2 ± 0.7 × 10 22 cm −2 respectively, as calculated from ISO/IRAS surface brightnesses. As Nagoya-4 m C 18 O (1-0) spectra show, these are dense molecular clouds with N(H) > 10 21 cm −2 column density peaks. The ISOPHOT 200 µm surface brightness is well correlated with the C 18 O line intensity (corr. coef . ≈ 70%). The large dust particle emissivity is found to be increased in the prototypical very dense core TMC-1 CP. As the low linewidths (∆v = 0.8 ± 0. 2k m s −1 ) indicate, the level of turbulent energy density is 50% lower in these three clouds than in other clouds of HCL2. Dense cores were identified inside the C 18 O clouds by NH3 measurements with the Effelsberg-100 m telescope and Nobeyama-45 m H 13 CO + data. The density of the dense cores is n ≥ 1.1 × 10 5 cm −3 ,a nd their kinetic temperatures are <10 K, in good agreement with the FIR results. The total molecular gas mass in the gravitationally bound cloud cores of TMC-1 CP and HCL2-E is about 21 Mand 8 Mrespectively. The cores, TMC-1 CPb and HCL2-Eab are associated with 3 low mass YSO candidate 2MASS point sources, while 35 other low mass YSO candidates are seen elsewhere in TMR south, which we consider as evidence for ongoing low mass star formation.

The infrared properties of the new outburst star IRAS 05436–0007 in quiescent phase

We compiled and investigated the infrared/sub-mm/mm SED of the new outburst star IRAS 05436-0007 in quiescent phase. The star is a flat-spectrum source, with an estimated total luminosity of L_bol ~ 5.6 L_sun, typical of low-mass T Tauri stars. The derived circumstellar mass of 0.5 M_sun is rather high among low-mass YSOs. The observed SED differs from the SEDs of typical T Tauri stars and of 4 well-known EXors, and resembles more the SEDs of FU Orionis objects indicating the presence of a circumstellar envelope. IRAS 05436-0007 seems to be a Class II source with an age of approximately 4x10^5 yr. In this evolutionary stage an accretion disk is already fully developed, though a circumstellar envelope may also be present. Observations of the present outburst will provide additional knowledge on the source.

New ideas on jets and outflows interactions

We propose an alternative global model for the flows surrounding both low and high mass YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infailing matter and follows a circulation pattern around the central object without necessarily being entrained by the jet. The model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We will try to clarify the relation between the fast jet and its surrounding molecular outflow, which does not primarily rely on entrainment (prompt or turbulent) in the model. Hence, there is no need to transfer a large momentum from the jet to the molecular outflow through the entrainment processes. The model suggests that radiative heating and the Poynting flux may ultimately be the main energy sources driving molecular outflow in addition to the entrainment processes by the fast jet.

Very Low Mass Stellar Populations in Star-Forming Regions: Near-Infrared Luminosity Functions and Mass Functions

We briefly describe recent studies of the low-mass young stellar populations including substellar objects and of their luminosity functions and mass functions, especially at lower-ends, in different star-forming regions. The mass function is determined by the technique based on the near-infrared photometry for estimating stellar luminosities and then translating them into stellar masses. We compare the local environmental characteristics of regions in which high-mass stars form with those of regions producing only low-mass stars and intermediate stars. We find that there exist numerous very low-mass YSO candidates including young brown dwarfs and young isolated objects with planetary masses in common. Further, the luminosity functions and mass functions in the star-forming regions might not have a uniform shape below the hydrogen-burning limit.

Multi-epoch water maser survey towards low-mass YSOs

We present the results from a series of multi-epoch 22 GHz H2O maser surveys with the Nobeyama 45 m telescope and the VLA towards low-mass young stellar objects, including all the class 0 sources in the northern sky. Our Nobeyama 45 m survey is the deepest survey - down to an isotopic H2O luminosity of ∼ 10−13L⊙ - performed so far. From this survey, we obtained the following results. (1) Class 0 sources show high H2O maser activity: our derived detection rates are ∼ 38% for class 0, but only ∼ 4% for class I sources. (2) Activity of the H2O masers is more likely related to 100 AU scale ionized jets than to large scale molecular outflows.

Chemistry in the Envelopes around Massive Young Stars

Recent observational studies of intermediate- and high-mass star-forming regions at submillimeter and infrared wavelengths are reviewed, and chemical diagnostics of the different physical components associated with young stellar objects are summarized. Procedures for determining the temperature, density and abundance profiles in the envelopes are outlined. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios, the abundances of evaporated molecules, and the fraction of heated ices with increasing temperature. Since these diverse phenomena involve a range of temperatures from &lt; 100 K to 1000 K, the enhanced temperatures must be communicated to both the inner and outer parts of the envelopes. This ‘global heating’ plausibly results from the gradual dispersion of the envelopes with time. Similarities and differences with low-mass YSOs are discussed. The availability of accurate physical models will allow chemical models of ice evaporation followed by ‘hot core’ chemistry to be tested in detail.

Photon Heating of Envelopes around Young Stellar Objects: An Explanation for CO [ITAL]J[/ITAL] = 6–5 Emission

We propose that the narrow 12CO and 13CO J = 6-5 emission observed toward many low-mass young stellar objects is produced in molecular material in the circumstellar envelope, which is heated by the 10,000 K radiation field generated in the inner part of the accretion disk. Ultraviolet photons traveling through the biconical cavity evacuated by the bipolar outflow are scattered by dust grains present in the low-density material in the cavity. These photons are not energetic enough to photodissociate H2 and CO, but can heat the envelope surrounding the cavity. The temperature structure and the CO excitation of this photon-dominated region are computed using two-dimensional Monte Carlo methods. It is found that the material is heated up to a few hundred K close to the cavity wall, and that the observed low-velocity mid-J CO emission can be well explained by our model for a wide range in envelope density and stellar luminosity. Emergent CO spectra are compared to observations of the embedded low-mass YSO IRAS 04361+2547 (TMR-1).

Continued decline in the 1-5 m brightness of B5 IRS1: 1990-93.

J H K L L′ & nbM photometry of B5 IRS1 in 1992 and 1993 shows that this low-mass YSO continues to fade in the near-infrared at an average rate of about 0.24 mag per year, and that by 1993 September the J to K luminosity had fallen to one-third of its 1988 December value. The change in luminosity continues to be virtually colour-independent between J and K, although the K –M colour is increasing with time. The changes are consistent with an increase in extinction by a dust population depleted of grains with radii smaller than ~0.4 μm, or with the short-wavelength emission (λ ≲ 3 µm) being dominated by light scattered in a changing circumstellar geometry.