Young Stellar Sources Research Articles

Mid-IR images of methanol masers and ultracompact HII regions

Methanol masers and UCHII regions trace massive star formation sites. We have undertaken a mid-IR survey of 17 regions containing methanol masers and UCHIIs in order to locate the young stellar sources associated with them. The images were obtained from 8.7 to 18.8 μm with the mid-IR camera CID (Salas et al. 2003) on the 2.1m telescope of the Observatorio Astronomico Nacional at San Pedro Martir (Baja California, Mexico). The images were taken with a scale 0.55″/pix and the mean PSF was 1.5-2.0″(FWHM) close to the diffraction limit. We report as an example in Fig. 1 (left panel) our 18.8μm contours of IRAS 06061+2151 superimposed to the 2MASS Ks image. A young cluster of at least 4 sources has been found centered on the IRAS source (Anandarao et al. 2004). We have found two mid-IR sources coinciding with the source #2 and #4 of Anandarao et al. (2004). The source #4 is at the center of two H2 knots and a high velocity molecular outflow. The mid-IR emission from #2 is extended and coincides with the UCHII and MSX source. The methanol maser is approximately 10″ south of the source #2. The SEDs of both sources are illustrated in Fig. 1 (right panel). The IR spectral indices of source #2 and #4 are α(IR)=1.9 and 2.2 respectively.

New insights into the nature of mid-infrared emission associated with massive star formation: disks and outflow

AbstractRecent observations in the mid-infrared (5-25 μm) of massive young stellar sources have yielded a surprising result: many show evidence of mid-infrared emission from outflows and jets. These observations correlate well with other larger-scale outflow indicators and their geometries, such as what is seen in shock-excited H2 and CO emission. In some cases these mid-infrared observations identify the local maser emission as outflow or jet related.

The Eye of the Tornado - an isolated, high-mass young stellar object near the Galactic Centre

We present infrared (AAT, UKIRT) and radio (VLA, SEST) observations of the Eye of the Tornado, a compact source apparently near the head of the Tornado Nebula. The near-infrared Br-gamma and He I lines are broad (FWHM 40 and 30 km/s, respectively) and have a line centre at Vlsr = -205 km/s. This corresponds to a feature at the same velocity in the 12CO J=1-0 line profile. The kinematic velocity derived from Galactic rotation places the Eye at the distance of the Galactic Centre (i.e. 8.5 kpc) and separated (probably foreground) from the Tornado Nebula. Four knots of emission are seen in the Br-gamma line and at 6 and 20 cm. Together with the flat radio spectral index, we confirm that the Eye contains ionized gas, but that this is embedded within a dense molecular core. The spectral energy distribution can be modelled as a two-component blackbody + greybody, peaking at far-IR wavelengths. The knots are UC HII regions, and the core contains a luminous (2 x 10^4 Lsun), embedded, massive young stellar source. We also propose a geometrical model for the Eye to account for both its spectral energy distribution and its morphology.

New detection of near-infrared H2line emission in AFGL 5157

Narrow-band H(2) v = 1-0 S (1) and broad-band K' imaging observations towards AFGL 5157 revealed a number of new H(2) knots associated with the NH(3) core in the region, implying that the multiple H(2) outflows are driven by a cluster of embedded point sources in the core. The shell around the infrared cluster is more clearly seen than before, showing a few of newly detected ridge-like features. A string of H(2) knots newly revealed along one lobe of the known CO outflow suggests a parsec-scaled jet emanating from a young source in the core. These H(2) outflows and jet, together with the known compact H II region, radio continuum source, and H(2)O masers further imply an extremely embedded cluster containing massive stars within it. The evidence shows that the massive young stars within the cluster are in a strong outflow stage of evolution, and together with the nearby infrared cluster in NGC1985, further confirms that star forming process propagates from the west to the east in AFGL 5157. The two types of H(2) line emission, knots and shell, may be indicators of two different stages of star formation in clusters: the shocked H(2) knots are driven by the young stellar sources embraced in the younger cluster, while the diffuse shell-like structure comes from UV photons of relatively evolved cluster. The near-infrared nebula in AFGL 5157 is found to be only associated with the evolved cluster but not observed in the embedded cluster.

The Great PV Cephei Outflow: A Case Study in Outflow‐Cloud Interaction

We present a set of detailed molecular-line maps of the region associated with the giant Herbig-Haro flow HH 315 from the young star PV Cephei, aimed at studying the outflow-cloud interaction. Our study clearly shows that the HH 315 flow is affecting the kinematics of its surrounding medium and has been able to redistribute considerable amounts of the surrounding medium-density (~103 cm-3) gas in its star-forming core as well at parsec-scale distances from the source. The single-dish observations include a map of the outflow in the 12CO (2-1) line, with a beam size of 27'', and more extended maps of the outflow region in the 12CO (1-0) and 13CO (1-0) lines, with 45'' and 47'' beam sizes, respectively. A companion paper published in this issue presents higher resolution (IRAM 30 m) observations and discusses their implications. The giant molecular outflow HH 315 is a highly asymmetric bipolar flow with a projected linear extent of about 2 pc. Our results indicate that the two outflow lobes are each interacting with the ambient medium in different ways. The southern (redshifted) lobe, with a mass of 1.8 M☉, interacts with a dense ambient medium, very close to the young stellar outflow source, and its kinetic energy is comparable to both the turbulent and gravitational binding energy of its host cloud (of order 1044 ergs). In addition, we find evidence that the southern lobe is responsible for the creation of a cavity in the 13CO emission. In contrast, the northern (mainly blueshifted) outflow lobe, with a total mass of 4.8 M☉, extends farther from PV Cep and interacts with ambient gas much less dense than the southern lobe. There is very little 13CO emission north of the outflow source, and the only prominent 13CO emission is a shell-like structure coincident with the outer edge of the northern lobe, about 1.2 pc northwest of PV Cep. It appears that the northern lobe of the HH 315 outflow has been able to push aside a substantial fraction of the gas in the area, piling it in a dense shell-like structure at its edges. In addition, we find that about 50% of the gas in the region of the northern lobe has been put into motion by the outflow and that the northern outflow lobe is responsible for a velocity gradient in the ambient gas.

Disks around hot stars in the Trifid nebula

We report on mid-IR observations of the central region in the Trifid nebula, carried out with ISOCAM in several broad-band infrared filters and in the low resolution spectroscopic mode provided by the circular variable filter. Analysis of the emission indicates the presence of a hot dust component (500 to 1000 K) and a warm dust component at lower temperatures (150-200 K) around several members of the cluster exciting the HII region, and other stars undetected at optical wavelengths. Complementary VLA observations suggest that the mid-IR emission could arise from a dust cocoon or a circumstellar disk, evaporated under the ionization of the central source and the exciting star of the nebula. In several sources the $9.7\mu m$ silicate band is seen in emission. One young stellar source shows indications of crystalline silicates in the circumstellar dust.

Very Low-Luminosity Objects in Star-Forming Regions

AbstractIn order to tackle the problems of low-mass end of the initial mass function (IMF) in star-forming regions and the formation mechanisms of brown dwarfs, we have conducted deep infrared surveys of nearby molecular clouds. We have found a significant population of very low-luminosity sources with IR excesses in the Taurus cloud and the Chamaeleon cloud core regions whose extinction corrected J magnitudes are 3 to 8 mag fainter than those of typical T Tauri stars in the same cloud. Some of them are associated with even fainter companions. Follow-up IR spectroscopy has confirmed for the selected sources that their photospheric temperature is around 2000 to 3000 K. Thus, these very low-luminosity young stellar sources are most likely very low-mass T Tauri stars, and some of them might even be young brown dwarfs.

Upper limits to the detection of ammonia from protoplanetary disks around HL Tauri and L1551-IRS 5

We present NH3(1, 1) and (2, 2) observations of the young stellar sources HL Tau and L1551-IRS 5 using the VLA in its B-configuration, which provides an angular resolution of about 0.4 arcsec (about 50 AU at 140 pc) at 1.3 cm wavelength. Our goal was to detect and resolve circumstellar molecular disks with radius of the order of 100 AU around these two sources. No ammonia emission was detected toward either of them. The 3-sigma levels were 2.7 mJy/beam and 3.9 mJy/beam for HL Tau and L1551-IRS 5, respectively, with a velocity resolution of about 5 km/s. With this nondetection, we estimate upper limits to the mass of the proposed protoplanetary molecular disks (within a radius of 10 AU from the central stars) on the order of 0.02/(X(NH3)/10 exp -8) solar mass for HL Tau and 0.1/(X(NH3)/10 exp -8) solar mass for L1551-IRS 5.

Near Infrared Polarimetry of Dark Clouds and Star Forming Regions - Two Micron Polarization Survey of T Tauri Stars

Infrared polarimetry is one of the most useful methods to delineate the magnetic field structure in dark clouds and star-forming regions, where the intracloud extinction is so large that optical polarimetry is inaccessible. We have been conducting a near-infrared polarization survey of background field stars and embedded sources toward nearby dark clouds and star-forming regions (Tamura 1988). Particularly, the magnetic field structure in the denser regions of the clouds are well revealed in Heiles Cloud 2 in Taurus, ρ Oph core, and NGC1333 region in Perseus (Tamura et al. 1987; Sato et al. 1988; Tamura et al. 1988). This survey also suggests an interesting geometrical relationship between magnetic field and star-formation: the IR polarization of young stellar sources associated with mass outflow phenomena is perpendicular to the magnetic fields. This relationship suggests a presence of circumstellar matter (probably dust disk) with its plane perpendicular to the ambient magnetic field. Combining with another geometrical relationship that the elongation of the denser regions of the cloud is perpendicular to the magnetic field, the geometry suggests that the cloud contraction and subsequent star-formation have been strongly affected by the magnetic fields. Thus, it is important to study the universality of such geometrical relationship between IR polarization of young stellar sources and magnetic fields. In this paper, we report the results on a 2 micron polarization survey of 39 T Tauri stars, 8 young stellar objects and 11 background field stars in Taurus dark cloud complex.

Multi-epoch radio continuum surveys of the Rho Ophiuchi dark cloud

view Abstract Citations (61) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multi-Epoch Radio Continuum Surveys of the Rho Ophiuchi Dark Cloud Stine, P. C. ; Feigelson, E. D. ; Andre, P. ; Montmerle, T. Abstract Radio-continuum surveys of 4 sq. deg of the ρ Ophiuchi cloud were conducted on four occasions at 1.4 GHz with the Very Large Array to study the variability of radio counterparts of over one hundred pre-main-sequence stars detected at X-ray, infrared, and optical wavelengths. These observations are supplemented with 5 GHz observations of the dense core region of the cloud and pointed observations of selected sources. This work significantly increases the sensitivity of a previous study by André, Montmerle, and Feigelson (1987), in which eight definite young stellar radio sources were uncovered. In this study, only one new reliable stellar source was found, located near the cloud core. The overwhelming majority (≡90%) of young stellar objects over a large area remain undetected to low flux-density levels (≡1 mJy). The failure to detect most PMS stars in ρ Ophiuchi and other star-formation regions may be explained by a sufficiently dense layer of circumstellar ionized material, such as a weak stellar wind, that absorbs nonthermal surface radio emission but is not itself detectable in radio. It seems that the stellar radio sources in the ρ Ophiuchi cloud are the prototypes of a new, rare class of young stellar objects. These sources are characterized by an absence of dense circumstellar ionized material and by extended magnetic structures with sizes ⪆3 stellar radii. Publication: The Astronomical Journal Pub Date: October 1988 DOI: 10.1086/114889 Bibcode: 1988AJ.....96.1394S Keywords: Early Stars; Ophiuchi Clouds; Pre-Main Sequence Stars; Sky Surveys (Astronomy); Stellar Flares; Very Large Array (Vla); Radio Emission; Stellar Activity; Stellar Mass; X Ray Sources; Astrophysics; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (220)

The geometry of anisotropic CO outflows

A simple geometrical model for the space motions of the bipolar high-velocity CO outflows in regions of recent, active star formation is proposed. It is assumed that the velocity field of the neutral gas component can be represented by large-scale uniform motions. From observations of the spatial distribution and from the characteristics of the line shape of the high-velocity molecular gas emission the geometry of the line-emitting regions can be inferred, i.e., the direction in space and the collimating angle of the flow. The model has been applied to regions where a check on presently obtained results is provided by independent optical determinations of the motions of Herbig-Haro objects associated with the CO flows. These two methods are in good agreement and, furthermore, the results obtained provide convincingly strong evidence for the physical association of CO outflows and Herbig-Haro objects. This also supports the common view that a young stellar central source is responsible for the active phenomena observed in its environmental neighborhood. It is noteworthy that within the framework of the model the determination of the flow geometry of the high-velocity gas from CO measurements is independent of the distance to the source and, furthermore, can be done atmore » relatively low spatial resolution. 32 references.« less

POLARIMETRY OF INFRARED SOURCES

Polarization at infrared wavelengths has been detected from a number of different objects within the Galaxy. These include young sources associated with molecular clouds and H II regions, cool stars with thick circumstellar shells, bi-polar nebulae, and normal stars suffering interstellar polarization. Typical levels of polarization detected at 2.2 μm are up to 25% for the molecular cloud sources, less than −5% for the cool stars, around 30% for some bi-polar nebulae and less than −2% for interstellar polarization. For the latter three types of source the origin of the polarization is basically understood: it results from scattering of stellar radiation off small particles in the surrounding shell or nebula in the cool stars and bi-polar nebulae and by transfer of flux through a foreground medium of aligned dust grains for the interstellar polarization. The phenomenon of large infrared polarization in the young stellar and pre-stellar sources is less well understood, and it is to this problem that we address ourselves in this review.