Intermediate-mass Young Stellar Objects Research Articles

INTRINSICALLY RED SOURCES OBSERVED BYSPITZERIN THE GALACTIC MIDPLANE

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg2 of the Galactic midplane, and consist mostly of high- and intermediate-mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position-dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50%-70% of YSOs and 30%-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2%-3% of all the red sources. 1004 red sources in the GLIMPSE II region, mostly AGB stars with high mass-loss rates, show significant (≥0.3 mag) variability at 4.5 and/or 8.0 μm. With over 11,000 likely YSOs and over 7000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate-mass YSOs in the Milky Way Galaxy.

Astrometry of H2O Masers in Nearby Star-Forming Regions with VERA III. IRAS22198+6336 in Lynds1204G

Abstract We present results of multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22GHz H$_{2}$O masers associated with a young stellar object (YSO) IRAS22198$+$6336 in a dark cloud, L1204G. Based on phase-referencing VLBI astrometry, we derived an annual parallax of IRAS22198$+$6336 to be 1.309$\pm$0.047mas, corresponding to the distance of 764$\pm$27pc from the Sun. Although the most principal error source of our astrometry is attributed to the internal structure of the maser spots, we successfully reduced the errors in the derived annual parallax by employing position measurements for all of the 26 detected maser spots. Based on this result, we reanalyzed the spectral energy distribution of IRAS22198$+$6336 and found that the bolometric luminosity and total mass of IRAS22198$+$6336 are 450$L_{\odot}$ and 7$M_{\odot}$, respectively. These values are consistent with an intermediate-mass YSO deeply embedded in the dense dust core, which has been proposed to be an intermediate-mass counterpart of a low-mass Class 0 source. In addition, we obtained absolute proper motions of the H$_{2}$O masers for the most blue-shifted components. We propose that the collimated jets aligned along the east-west direction are the most plausible explanation for the origin of the detected maser features.

Warm molecular gas in the envelope and outflow of IRAS 12496-7650 (DK Chamaeleontis)

Aims. To obtain insight into the physical structure of the warm gas in the inner envelope of protostars and the interaction with the outflow. Methods. Sub-millimeter observations of 12 CO, 13 CO and/or C 18 O in J = 3-2, J = 4-3 and J = 7-6 were obtained with the APEX Telescope towards IRAS 12496-7650, an intermediate mass young stellar object. The data are compared to ISO-LWS observations of CO J = 14-13 up to J = 19-18 lines to test the different proposed origins of the CO lines. Results. The outflow is prominently detected in the 3-2 and 4-3 lines, but not seen at similar velocities in the 7-6 line, constraining the temperature in the high-velocity (>5 km s -1 from line center) gas to less than 50 K, much lower than inferred from the analysis of the ISO-LWS data. In addition, no isothermal gas model can reproduce the emission in both the 7-6 and the higher-J ISO-LWS lines. The 7-6 line probably originates in the inner (<250 AU) region of the envelope at ∼150 K. Detailed radiative transfer calculations suggest that the ISO-LWS lines are excited by a different mechanism, possibly related to the larger-scale outflow. All possible mechanisms on scales smaller than 8 are excluded. High-resolution continuum as well as high-J 12 CO and isotopic line mapping are needed to better constrain the structure of the warm gas in the inner envelope and the interaction with the outflow.

Detection of a hot core in the intermediate-mass Class 0 protostar NGC 7129–FIRS 2

We report high angular resolution (HPBW ∼ 0.6 �� × 0.5 �� at 1.3 mm) observations of the Class 0 intermediate-mass (IM) protostar NGC 7129- FIRS 2 using the Plateau de Bure Interferometer. Our observations show the existence of an intense unresolved source in the continuum at 1.3 mm and 3 mm at the position of the Class 0 object. In addition, compact CH3CN emission is detected at this position. The high rotational temperature derived from the CH3CN lines (Trot ≈ 50 K), as well as the enhanced CH3CN fractional abundance (X(CH3CN) ∼ 7.0×10 −9 ), shows the existence of a hot core in this IM young stellar object. This is to our knowledge the first IM hot core detected so far. Interferometric maps of the region in the CH3OH 5kk� →4kkand D2CO 404→303 lines are also presented in this paper. The methanol emission presents two condensations, one associated with the hot core, which was very intense in the high upper state energy lines (Eu > 100 K), and the other associated with the bipolar outflow which dominates the emission in the low excitation lines. Enhanced CH3OH abundances (X(CH3OH) ∼ 3 × 10 −8 -af ew 10 −7 ) were measured in both components. While intense D2CO 404 → 303 emission was detected towards the hot core, the N2D + 3 → 2 line was not detected in our interferometric observations. The different behaviors of D2CO and N2D + emissions suggest different formation mechanisms for the two species and different deuteration processes for H2CO and N2H + (surface and gas-phase chemistry, respectively). Finally, the spectrum of the large bandwidth correlator shows a forest of lines at the hot core position, revealing that this object is extraordinarily rich in complex molecules. For deeper insight into the chemistry of complex molecules, we compared the fractional abundances of the complex O- and N- bearing species in FIRS 2 with those in hot corinos and massive hot cores. Within the large uncertainty involved in fractional abundance estimates towards hot cores, we did not detect any variation in the relative abundances of O- and N-bearing molecules ((CH3CN)/(CH3OH)) with the hot core luminosity. However, the O-bearing species H2CO and HCOOH seemed to be more abundant in low and intermediate mass stars than in massive star-forming regions. We propose that this could be the consequence of a different grain mantle composition in low and massive star-forming regions.

A circumstellar disk associated with a massive protostellar object

The formation process for stars with masses several times that of the Sun is still unclear. The two main theories are mergers of several low-mass young stellar objects, which requires a high stellar density, or mass accretion from circumstellar disks in the same way as low-mass stars are formed, accompanied by outflows during the process of gravitational infall. Although a number of disks have been discovered around low- and intermediate-mass young stellar objects, the presence of disks around massive young stellar objects is still uncertain and the mass of the disk system detected around one such object, M17, is disputed. Here we report near-infrared imaging polarimetry that reveals an outflow/disk system around the Becklin-Neugebauer protostellar object, which has a mass of at least seven solar masses (M(o)). This strongly supports the theory that stars with masses of at least 7M(o) form in the same way as lower mass stars.

Chemical evolution in the environment of intermediate mass young stellar objects

the date of receipt and acceptance should be inserted later Abstract. We have carried out a molecular survey of the Class 0 IM protostar NGC 7129 - FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkH� 234 with the aim of studying the chemical evolution of the envelopes of intermediate-mass (IM) young stellar objects (YSOs). Both objects have similar luminosities (�500 L⊙) and are located in the same molecular cloud which minimizes the chemical differences due to different stellar masses or initial cloud conditions. Moreover, since they are located at the same distance, we have the same spatial resolution in both objects. A total of 17 molecular species (including rarer isotopes) have been observed in both objects and the structure of their envelopes and outflows is determined with unprecedent detail. Our results show that the protostellar envelopes are dispersed and warmed up during the evolution to become a pre-main sequence star. In fact, the envelope mass decreases by a factor >5 from FIRS 2 to LkH� 234, while the kinetic temperature increases from �13 K to 28 K. On the other hand, there is no molecular outflow associated with LkH� 234. The molecular outflow seems to stop before the star becomes visible. These physical changes strongly affect the chemistry of their envelopes. The N2H + and NH3 abundances seem to be quite similar in both objects. However, the H 13 CO + abundance is a factor of �3 lower in the densest part of FIRS 2 than in LkH� 234, very likely because of depletion. In contrast, the SiO abundance is larger by a factor of �100 in FIRS 2 than in LkH� 234. CS presents a complex behavior since its emission arises in different envelope components (outflow, cold envelope, hot core) and could also suffer from depletion. The CH3OH and H2CO column densities are very similar in FIRS 2 and LkH� 234 which implies that the beam averaged abundances are a factor >5 larger in LkH� 234 than in FIRS 2. The same case is found for the PDR tracers CN and HCN which have similar column densities in both objects. Finally, a complex behavior is found for the deuterated compounds. While the DCO + /H 13 CO + ratio decreases by a factor of �4 from FIRS 2 to LkH� 234, the D2CO/H2CO ratios is within a factor 1.5 in both objects. The detection of a warn CH3CN component with Tk>63 K shows the existence of a hot core in FIRS 2. Thus far, only a handful of hot cores have been detected in low and intermediate mass stars. Based on our results in FIRS 2 and LkH� 234, we propose some abundance ratios that can be used as chemical clocks for the envelopes of IM YSOs. The SiO/CS, CN/N2H + , HCN/N2H + , DCO + /HCO + and D2CO/DCO + ratios are good diagnostics of the protostellar evolutionary stage.

X-Ray Observation on the Monoceros R2 Star-Forming Region with the Chandra ACIS-I Array

Abstract We report on the results of the Chandra ACIS-I observation on the central region of the Monoceros R2 cloud (Mon R2), a high-mass star-forming region (SFR) at a distance of 830 pc. With a deep exposure of $\sim 100 \,\mathrm{ks}$, we detected 368 X-ray sources, $\sim 80\%$ of which were identified with the near-infrared (NIR) counterparts. We systematically analyzed the spectra and time variability of most of the X-ray emitting sources and provided a comprehensive X-ray source catalog for the first time. Using the $J$-, $H$-, and $K$-band magnitudes of the NIR counterparts, we estimated the evolutionary phase (classical T Tauri stars and weak-lined T Tauri stars) and the mass of the X-ray emitting sources, and analyzed the X-ray properties as a function of the age and mass. We found a marginal hint that classical T Tauri stars have a slightly higher temperature (2.4 keV) than that of weak-lined T Tauri stars (2.0 keV). A significant fraction of the high- and intermediate-mass sources have a time variability and high plasma temperatures (2.7 keV) similar to those of low-mass sources (2.0 keV). We performed the same analysis for other SFRs, the Orion Nebula Cluster and Orion Molecular Cloud-2/3, and obtained similar results to Mon R2. This supports the earlier results of this observation obtained by Kohno et al. (2002, ApJ, 567, 423) and Preibisch et al. (2002, A&amp;A, 392, 945) that high- and intermediate-mass young stellar objects emit X-rays via magnetic activity. We also found a significant difference in the spatial distribution between X-ray and NIR sources.

Detection of Crystalline Silicates around the T Tauri Star Hen 3-600A

We have carried out mid-infrared N-band spectroscopic observations of the T Tauri star Hen 3-600A in the TW Hydra association with the COMICS on the 8.2m Subaru Telescope and found structured features in its spectrum. These structured features are well explained by a combination of crystalline forsterite, crystalline enstatite, silica and glassy olivine grains. Among intermediate-mass young stellar objects (YSOs), crystalline silicates have already been detected, but no firm detection has been reported so far for low-mass YSOs such as T Tauri stars. This is the first clear detection of crystalline silicates in low-mass YSOs and shows that the crystallization event occurs even in the protoplanetary disk of low-mass YSOs in the T Tauri phase. The physical processes leading to the inferred dust composition in the Hen3-600A system may be analogous to those occured in the early epoch of the Solar system.

The young intermediate-mass stellar object AFGL 490 – A disk surrounded by a cold envelope

AFGL 490 is a key target of the class of deeply embedded intermediate-mass young stellar objects in a transition stage to Herbig Be stars (L = 2.2-4.0 × 10 3 L� ). In this paper, we present a comprehensive set of single-dish line data which characterize the envelope of the source. In addition, observations of CS J = 2→1 and the corresponding continuum at 97.98 GHz have been obtained with the Plateau de Bure (PdB) interferometer, which are sensitive to the small-scale structure around the stellar source. The PdB line data show a bar-like elongated gas structure of 22000 AU × 6000 AU size with a position angle of ≈-45 ◦ . This bar represents the flattened inner envelope surrounding a disk-like structure (radius ≤500 AU) for which we find evidence very close to the young B star. Due to strong (self-)absorption in the velocity range vlsr = -12.5 ... -15 km s −1 , only the outer line wings can be used to study the gas motion. Maps of the integrated red and blue line wing emission show two well-separated gas blobs around AFGL 490, which are interpreted as a disk. The 3 mm continuum interferometer map shows a point source at the position of AFGL 490 with a flux of 240 mJy. This flux translates into a total mass of 3-6 Mof the disk which is comparable to the stellar mass of about 8 M� . This configuration is unstable and will disappear in 10 3 -10 4 years due to gravitational instabilities. Photometric data from ISOPHOT and spectroscopic data from ISO-SWS have been obtained. Together with submillimetre continuum data a very complete spectral energy distribution of the envelope could be compiled. Analysis of the data shows that the central region of AFGL 490 has a steeper density gradient compared with the outer molecular envelope. All data clearly point to a low temperature (25-35 K) of this envelope. To determine the chemical state of the object, we determined the abundances of 13 molecules towards AFGL 490. The molecular line and ISO-SWS data are used to derive the gas-solid abundance ratios for H2O, CO, and CO2. The chemical results, such as the relatively low gas-to-solid ratios and the low CH3OH excitation, emphasize the presence of a cold molecular envelope. We found evidence for other outflow systems in the envelope around AFGL 490. Red-shifted and blue-shifted gas blobs with a separation of about 20000 AU were detected. Their centre is located roughly 3 �� to the south of AFGL 490, and their morphology implies that a deeply embedded low-mass object drives a jet which enters the denser envelope material at such a large distance. Two further outflow systems in the close neighbourhood of AFGL 490 could be identified. All these data point to the formation of a group of low-mass stars around AFGL 490. It is very remarkable that these outflows do not influence the global physical and chemical structure of the envelope.

Diffraction-limited bispectrum speckle interferometry of the Herbig Be star R Mon

We explore the structures immediately surrounding the intermediate-mass young stellar object R Mon with bispectrum speckle interferometry, conventional near-infrared imaging and by analyzing optical HST archive data. Our near-infrared speckle images with unprecedented diffraction-limited resolution of 55 mas (~44 AU; H -band) and 76 mas (~61 AU; K -band) represent the highest resolution R Mon images obtained so far and exhibit previously unseen complex structures. While the binary companion R Mon B appears as an unresolved point source in our speckle images, the image of the primary R Mon A is marginally extended in the K -band and significantly extended in the H -band. The most prominent new feature is a bright arc-shaped structure, pointing away from R Mon in north-western direction. We interpret this feature as the surface of a dense structure near the thick circumstellar disk or torus around R Mon. Our images also reveal several twisted filaments of helical shape which are similar to the twisted filaments in the outer parts of the nebula. We identify structures which probably are responsible for casting pronounced shadows in the outer regions of the NGC 2261 reflection nebula. Finally, we discuss the relation of the observed features, in particular the arc-shaped speckle feature, to the wind and outflow activity (Herbig-Haro objects and jets) of R Mon.

Magnetic activities on young stars

Almost all young stellar objects (YSOs) are now known to emit X-rays driven by the magnetic activity. The solar-type dynamo is the plausible mechanism to generate the magnetic fields, but it does not work properly on protostars or intermediate-mass YSOs due to the difference of their stellar inner structure. To find an alternative mechanism, the structure and energy of magnetic fields surrounding stars should be investigated. In this sence, a stellar X-ray flare gives us important information because it directly traces an activity of one magnetic loop. With this motivation, we searched for stellar X-ray flares using the ASCA satellite and detected seven flares from low-mass and intermediate-mass YSOs. By comparing flare parameters with the absorption column density (NH), we found that younger stars tend to have higher plasma temperature and larger emitting volumes. Furthermore, the flare loop size should be far larger than the solar radii in low-mass protostars and Herbig Ae/Be stars (HAEBEs). These facts suggest that the magnetic fields of low-mass protostars and HAEBEs far extend from the stellar main body and probably connect to the circumstellar disks, and that the magnetic reconnection by the star-disk differential rotation should energize the X-ray flares. We also present supporting evidence of quasi-periodic flares detected on the ClassI protostar YLW15.

Possible Infall around the Intermediate-Mass Young Stellar Object NGC 2264 IRS

Analysis of multitransitional CS and CO observations toward the class I intermediate-mass young stellar object NGC 2264 IRS indicates that the outflow associated with this source dominates the kinematic structure of the CS J = 2 → 1, J = 3 → 2, and J = 5 → 4 spectra. For example, toward the source, centroid velocities (the first moments) of these line profiles increase with increasing optical depth; i.e., the emission becomes increasingly redshifted, which is expected for outflow from a central source. In contrast, the CS J = 7 → 6 spectrum toward NGC 2264 IRS is asymmetric, having a single blueshifted peak with a red shoulder, which suggests the possible presence of infall toward NGC 2264 IRS. In addition, the centroid velocities for the CS J = 7 → 6 line become more negative (lines are blueshifted) as one approaches the source perpendicular to the outflow. Furthermore, the CS J = 7 → 6 line is blueshifted relative to the CS J = 2 → 1, J = 3 → 2, and J = 5 → 4 lines toward NGC 2264 IRS. Integrated intensity contours of the CS J = 5 → 4 emission are extended in the direction of the outflow; however, the CS J = 7 → 6 emission is extended in the direction perpendicular to the outflow. We describe a preliminary attempt to model the CS J = 7 → 6 spectra with a Monte Carlo radiative transfer code (Choi et al.) using an inside-out collapse scenario (Shu).

Search for H2 Emission from Disks Around T Tauri and Herbig Ae Stars

Preliminary results are presented of observations of the pure rotational lines of H2 toward T Tauri and Herbig Ae stars using the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). The sources are selected to be isolated low- and intermediate-mass young stellar objects, for which the presence of a circumstellar disk has been established by millimeter interferometry. The lowest H2 S(0) and S(1) lines are detected in 3 out of 5 objects. The measured intensities indicate ∼ 0.01 M⊙ of warm (T ≈ 150 K) gas in the ISO beam. It is argued that for at least one case (HD 163296), the emission is probably dominated by the warm gas in the circumstellar disk rather than by shocked- or photon-heated gas in the surrounding envelope. Such observations can provide important constraints on the radial and vertical temperature profiles in circumstellar disks.

Near-infrared discovery of the L810 nebula illuminator

view Abstract Citations (16) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Near-Infrared Discovery of the L810 Nebula Illuminator Yun, Joao L. ; Clemens, Dan P. ; McCaughrean, Mark J. ; Rieke, Marcia Abstract As part of a deep NIR imaging survey for IR point sources in Bok globules, we have discovered the illuminator of the optical reflection nebula seen toward the Bok globule L810. The illuminator (L810 IRS) is located at the position predicted by Scarrott et al. (1991) from analysis of their optical polarimetry. This source is very red (J-K about 3.9) and bright (mK = 10.7), and has an estimated bolometric luminosity of 890 solar luminosities. A NIR nebula was found which extends 20 arcsec symmetrically to the north and to the south of L810 IRS, exhibiting an hourglass shape. Both the morphology and the orientation of the NIR nebula are in good agreement with the morphology and orientation of the high-velocity molecular outflow associated with L810. Finally, we found a NIR jetlike structure whose axis is collinear with L810 IRS. Together, these findings indicate that L810 IRS is likely to be a deeply embedded, very active intermediate-mass young stellar object within the L810 cloud. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/186841 Bibcode: 1993ApJ...408L.101Y Keywords: Infrared Imagery; Infrared Stars; Interstellar Matter; Point Sources; Reflection Nebulae; Star Formation; Infrared Astronomy; Near Infrared Radiation; Astrophysics; STARS: FORMATION; STARS: PRE--MAIN-SEQUENCE; ISM: INDIVIDUAL ALPHANUMERIC: L810; ISM: JETS AND OUTFLOWS; ISM: REFLECTION NEBULAE; INFRARED: STARS full text sources ADS | data products SIMBAD (3)

Interpreting infrared color-color diagrams - Circumstellar disks around low- and intermediate-mass young stellar objects

Interpreting infrared color-color diagrams - Circumstellar disks around low- and intermediate-mass young stellar objects