Low-luminosity Young Stellar Objects Research Articles

A Search for Water Maser Emission from Brown Dwarfs and Low-luminosity Young Stellar Objects

Abstract We present a survey for water maser emission toward a sample of 44 low-luminosity young objects, comprising (proto-)brown dwarfs, first hydrostatic cores (FHCs), and other young stellar objects (YSOs) with bolometric luminosities lower than 0.4 L ⊙. Water maser emission is a good tracer of energetic processes, such as mass-loss and/or accretion, and is a useful tool to study these processes with very high angular resolution. This type of emission has been confirmed in objects with L bol ≳ 1 L ⊙. Objects with lower luminosities also undergo mass-loss and accretion, and thus, are prospective sites of maser emission. Our sensitive single-dish observations provided a single detection when pointing toward the FHC L1448 IRS 2E. However, follow-up interferometric observations showed water maser emission associated with the nearby YSO L1448 IRS 2 (a Class 0 protostar of L bol ≃ 3.6–5.3 L ⊙) and did not find any emission toward L1448 IRS 2E. The upper limits for water maser emission determined by our observations are one order of magnitude lower than expected from the correlation between water maser luminosities and bolometric luminosities found for YSOs. This suggests that this correlation does not hold at the lower end of the (sub)stellar mass spectrum. Possible reasons are that the slope of this correlation is steeper at L bol ≤ 1 L ⊙ or that there is an absolute luminosity threshold below which water maser emission cannot be produced. Alternatively, if the correlation still stands at low luminosity, the detection rates of masers would be significantly lower than the values obtained in higher-luminosity Class 0 protostars.

RADIO PROPERTIES OF YOUNG STELLAR OBJECTS IN THE CORE OF THE SERPENS SOUTH INFRARED DARK CLOUD

ABSTRACT We present deep radio continuum observations of the star-forming core of the Serpens South Infrared Dark Cloud with the Karl G. Jansky Very Large Array. Observations were conducted in two bands centered at 7.25 GHz (4.14 cm) and 4.75 GHz (6.31 cm) with a &sgr; rms ?> of 8.5 and 11.1 μJy/beam, respectively. We also use 2MASS, Spitzer and Herschel data to put our radio observations in the context of young stellar populations characterized by near and far-infrared observations. Within a 5′ × 5′ region of interest around the central cluster, we detect roughly eighteen radio sources, seven of which we determine are protostellar in nature due to their radio spectral indices and their association with infrared sources. We find evidence for a previously undetected embedded Class 0 protostar and reaffirm Class 0 protostellar classifications determined by previous millimeter wavelength continuum studies. We use our infrared data to derive mid-infrared luminosities for three of our protostellar sources and find relative agreement between the known young stellar object (YSO) radio luminosity versus bolometric luminosity correlation. Lastly, we marginally detect an additional six radio sources at the 2–3σ level that lie within two arcseconds of infrared YSO candidates, providing motivation for higher sensitivity studies to clarify the nature of these sources and further probe embedded and/or low luminosity YSOs in Serpens South.

Radio Emission From Low Mass Young Stellar Objects

AbstractCompact radio emission provides a reliable method for the detection of low luminosity young stellar objects (YSOs), and is particularly useful for detecting the earliest stages of protostellar evolution where the source itself may still be heavily embedded in its natal dust envelope. For such Class 0 and Class I objects the dominant radio emission mechanism is expected to be free-free, however unlike massive YSOs the way in which this radio emission is produced remains a subject of debate. As larger samples of radio YSOs become available the relationship between the radio luminosity of the Class 0/I population and their wider global properties is now being clarified. Furthermore, the broader scientific applications of such samples are also becoming increasingly apparent. These improved constraints on the nature of the radio emission from YSOs are now contributing to our understanding of not only the evolutionary physics of protostars themselves but also their wider impact on their surroundings. He...

RADIO IMAGING OF THE NGC 1333 IRAS 4B REGION

The NGC 1333 IRAS 4B region is observed in the 6.9 mm and 1.3 cm continuum with an angular resolution of about 0.4 arcseconds. IRAS 4BI is detected in both bands, and BII is detected in the 6.9 mm continuum only. The 1.3 cm source of BI seems to be a disk-like flattened structure with a size of about 50 AU. IRAS 4BI does not show any sign of multiplicity. Examinations of archival infrared images show that the dominating emission feature in this region is a bright peak in the southern outflow driven by BI, corresponding to the molecular hydrogen emission source HL 9a. Both BI and BII are undetectable in the mid-IR bands. The upper limit on the far-IR flux of IRAS 4BII suggests that it may be a very low luminosity young stellar object.

Optical Spectroscopy of Candidates of Young Stellar Objects in NGC 1333

Abstract We carried out low-resolution optical spectroscopy of 14 low-luminosity young stellar object (YSO) candidates in the NGC 1333 cluster. These objects were previously identified by a near-infrared imaging survey. Eleven objects were confirmed to be YSOs by the H$\alpha$ line emission. The strengths of the H$\alpha$ emission are correlated with the near-infrared excesses of the objects. The spectral types of all YSOs are estimated to be M-type, indicative of low mass. Comparisons of the results of our spectroscopic observations and previous photometric observations with evolutionary tracks on the HR diagram suggest two objects to be young brown dwarfs.

YOUNG BROWN DWARFS IN THE CORE OF THE W3 MAIN STAR-FORMING REGION

We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR observations with the Subaru telescope, to search for very low mass young stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 region and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color (CC) and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low luminosity YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. Based on the comparison between theoretical evolutionary models of very low-mass PMS objects with the observed CM diagram, we find there exists a substantial substellar population in the observed region. The mass function (MF) does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the MF slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.

Near-Infrared Spectroscopy of Very Low-Luminosity Young Stellar Objects in the Taurus Molecular Cloud

Abstract We carried out near-infrared spectroscopic observations of 23 very low-luminosity young stellar object (YSO) candidates and 5 of their companions in Heiles Cloud 2, one of the densest parts of the Taurus molecular cloud. Twelve objects were confirmed to be YSOs by the $\mathrm{B}\gamma$ feature. The effective temperatures of the YSOs and of the companions are estimated from the 2.26-${\mu \mathrm {m}}$ feature, the 2.21-${\mu \mathrm {m}}$ feature, and the $\mathrm{H}_{2}\mathrm{O}$ band strengths. Detailed comparisons of our photometric and spectroscopic observations with evolutionary tracks on the HR diagram suggest some objects to be very low-mass YSOs.

Spectroscopy of Very Low Luminosity Young Stellar Objects in Taurus

I present low-resolution optical spectroscopy of two very low luminosity young stellar object candidates in Taurus. Both have faint companions with separations less than 6'', reported by Itoh, Tamura, & Nakajima, who considered them as likely substellar objects. However, the optical spectra demonstrate that they are reddened background stars (Av ≥ 4 mag). One of the candidate primaries is confirmed as a new member of the Taurus T Tauri association. It is a classical T Tauri star of early M spectral type with blue and near-infrared continuum excesses. The spectrophotometric properties indicate that it could be an edge-on disk. The other candidate primary is a background early-type star.

The Complex Protostellar Source IRAS 04325+2402

We report Hubble Space Telescope near-infrared NICMOS observations of a remarkable low-luminosity Class I (protostellar) source in the Taurus Molecular Cloud. IRAS 04325+2402 exhibits a complex bipolar scattered light nebula. The central continuum source is resolved and may be multiple, or it may be crossed by a small dust lane. Complex arcs seen in scattered light surround the central source; the physical nature of these structures is not clear, but they may reflect perturbations from multiple stellar sources or from time-dependent mass ejection. A second, resolved continuum source is found at a projected distance of approximately 1150 AU from the central region, near the edge of a nebular lobe probably produced by outflow. The images indicate that this second source is another low-luminosity young stellar object, seen nearly edge-on through a dusty disk and envelope system with disk diameter of about 60 AU. We suggest that the scattered light streaks associated with this second source are limb-brightened outflow cavities in the dusty envelope, possibly perturbed by interaction with the outflow lobes of the main source. The nature of the companion is uncertain, since it is observed mostly in scattered light. But it is most probably a very low mass star or brown dwarf, with a minimum luminosity of approximately 10-2 L⊙. Our results show that protostellar sources may have multiple centers of infall and nonaligned disks and outflows, even on relatively small scales.

A Near-Infrared Search for Companions around Very Low Luminosity Young Stellar Objects in Taurus

We have carried out a near-infrared search for companions around 23 very low luminosity young stellar objects (YSOs) in the Taurus molecular cloud. After sophisticated photometry and analysis, five extremely low luminosity YSO (ELL-YSO) candidate companions were identified by both their near-infrared colors and proximity to the primary (separation less than 6''). They show infrared excess, as do ordinary YSOs. Their absolute J-band magnitudes range from 9 to 12 mag. The masses of these companions are estimated from their J-band luminosities, which use recent evolutionary tracks for very low mass objects. It is found that all are young brown dwarf candidates. The Ks-band magnitude difference between the ELL-YSO candidate companions and their primaries ranges from 2 to 6 mag, significantly larger than observed in T Tauri binaries. It is suggested that the companions are formed by the fragmentation of a disk around the primary. The binary frequency is 22% for systems with a period ranging between 107–108 days. This frequency is consistent with that of T Tauri stars, but it is significantly higher than that of low-mass main-sequence stars.

Isolated and companion young brown dwarfs in the taurus and chamaeleon molecular clouds

Infrared imaging observations have detected a dozen faint young stellar objects (YSOs) in the Taurus and Chamaeleon molecular clouds whose near-infrared colors are similar to those of classical T Tauri stars (TTS). They are around four magnitudes fainter than low-luminosity YSOs in Taurus detected in earlier surveys and as much as eight magnitudes fainter than typical TTS. The extreme faintness of the objects and their lower luminosity relative to previously identified brown dwarfs in the Pleiades indicate that these faint YSOs are very young brown dwarfs on the order of 1 million years old.

Thermal Radio Emission from Disk‐driven Centrifugal Winds

Disk-driven centrifugal winds can produce considerable bremsstrahlung radiation that could account for the radio observations of many low-luminosity young stellar objects. The wind is generated by ions that are flung out centrifugally along magnetic field lines threading the disk. Collisional drag between ions and neutrals heats the gas, giving rise to a hot (T ~ 104 K), partially ionized (nH+/nH ~ 0.1) region in the wind. The thermal radio emission from this hot region is computed for several wind models, and it is shown that the flux in the radio regime (1 v 20 GHz) is consistent with the observed values of, and upper limits on, the flux from classical T Tauri stars. Synthetic radio maps are calculated for different observed inclinations and are found to be compatible with the observed morphologies. Inclinations near 90° (edge-on to the disk) result in elongated radio maps, whereas smaller inclinations yield more circular contours. The models considered in this study have outflow rates of ~10–7 M☉ yr–1 tron densities in the range inferred from observations of forbidden lines. In particular, a general parameter study within the context of the model indicates that electron densities from ~ 107 to ~ 1012 cm–3 at a distance of ~ 1 AU from the central protostar are required to produce the range in observed radio luminosities and spectral shapes. The calculated wind models turn out to be partially optically thick, implying that any peristellar nonthermal emission would in general be unobservable. This could explain why classical T Tauri stars lack the nonthermal emission found in weak-lined T Tauri stars.

The Unusually Rich Infrared Emission-Line Spectrum of a Deeply Embedded Low-Luminosity Young Stellar Object

The Unusually Rich Infrared Emission-Line Spectrum of a Deeply Embedded Low-Luminosity Young Stellar Object

Twin Herbig-Haro Jets and Molecular Outflows in L1228

view Abstract Citations (56) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Twin Herbig-Haro Jets and Molecular Outflows in L1228 Bally, John ; Devine, David ; Fesen, Robert A. ; Lane, Adair P. Abstract We present new optical, near-infrared, and millimeter wavelength observations of the dark cloud L1228 located in the Cepheus Flare. There are at least two active young stellar objects within a 2' diameter region in the L1228 cloud core that drive two separate Herbig-Haro (HH) outflows, a molecular hydrogen jet, and associated CO outflows. The red nebulous object RNO 129 (HH 198), located 1 north of the L1228 cloud core, is a Herbig-Haro flow from a T Tauri star located near the eastern end of the HH object. The Herbig-Haro object HH 199 is associated with a large molecular outflow (L1228A) centered on a low-luminosity (4 Lsun) young stellar object IRAS 20582+7724, the brightest infrared source in this cloud core. The brightest visual wavelength components, HH 199R1 and R2, lie about 8' to the southwest of the IRAS source, where the redshifted lobe of the CO outflow may be breaking out of the molecular cloud. A chain of faint blueshifted (V ≍-40 to -150 km s-1) HH objects, HH 199B1 through HH 199B6, lies to the northeast of the IRAS source toward the blueshifted lobe of the L1228A CO outflow. The projected distance between HH 199R2 and HH 199B6 is about 20' or nearly 2 pc. Near the center of this outflow, a chain of knots bright in the 2.122 microns wavelength 5(1) line of molecular hydrogen extends for about 1' along a nearly east-west axis on both sides of IRAS 20582+7724. This axis differs from that of the L1228A/HH 199 outflow by about 40°. We consider a model in which the jet from IRAS 20582+7724 varies in its ejection direction. At present, the outflow as traced by the H_{2 }jet appears to be impacting dense C3H_{2 }emitting clumps east and west of the IRAS source. A redshifted CO ridge extends 10' to the west of the IRAS source at the same orientation as the H2 jet, but a more prominent northeast-southwest ridge extends along the HH 199 optical axis from HH 199B6 in the northeast, through the IRAS source, to HH 199R2 in the southwest. A periodic blueshifted (V ≍ -40 to -150 km s-1) chain of Herbig-Haro objects, HH 200, is superimposed on the redshifted lobe of the L1228A outflow. It is associated with the low-velocity blueshifted CO lobe of a second outflow, L1228B. The source of this jet is an embedded T Tauri star located 1'S northwest of IRAS 20582+7724. There is a faint HH object 9' to the northeast of the source of the HH 200 jet, directly opposite the location of the bright bow shock at the end of the blueshifted lobe of the HH 200 jet. This lobe may mark the location of an invisible counterjet HH 200 has a projected length of over 18' corresponding to 1.8 pc (assuming a distance of 300 pc to L1228), and it shows quasi-periodic knot spacing of roughly 1' corresponding to a projected spacing of 0.09 pc. At a flow velocity of 300 km s-1, and an assumed outflow inclination angle of 45°, this spacing may be a result of episodes of enhanced mass loss occurring roughly every 500 years. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176486 Bibcode: 1995ApJ...454..345B Keywords: INFRARED: ISM: LINES AND BANDS; ISM: INDIVIDUAL ALPHANUMERIC: L1228; ISM: JETS AND OUTFLOWS; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (26)

Radio continuum emission from young stellar objects in L1641

view Abstract Citations (32) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Continuum Emission from Young Stellar Objects in L1641 Morgan, James A. ; Snell, Ronald L. ; Strom, Karen M. Abstract The results of a 6 and 20 cm radio continuum survey of young stellar objects in the L1641 region located south of the Orion Nebula are presented. Four are identified as low-luminosity young stellar objects in L1641 and three more as Herbig-Haro or Herbig-Haro-like objects. These objects have bolometric luminosities between 80 and 300 solar, and their 6-20 cm spectral index suggests optically thick, free-free emission. They are characterized by a rising spectrum between 2.2 and 25 microns, have no optical counterparts, and are associated with stellar wind activity. Thus, detectable radio continuum emission may be produced only by the youngest and most luminous objects in L1641. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169264 Bibcode: 1990ApJ...362..274M Keywords: Continuous Radiation; Early Stars; Orion Nebula; Pre-Main Sequence Stars; Radio Emission; Stellar Radiation; Extragalactic Radio Sources; Herbig-Haro Objects; Stellar Luminosity; Stellar Winds; Astrophysics; NEBULAE: INDIVIDUAL ALPHANUMERIC: L1641 (LYNDS 1641); STARS: PRE--MAIN-SEQUENCE; STARS: RADIO RADIATION full text sources ADS | data products SIMBAD (58)

A Remarkable Bipolar Flow in the Center of the Rho Ophiuchi Cloud

Using the IRAM 30-m telescope in August and December 1988, we have discovered the first molecular outflow in the central part (L1688) of the nearby ρ Ophiuchi dark cloud. This outflow, found in the J = 2 — 1 line of 12CO near the cloud core A, is an extreme case, weak (outflow mass-loss rate ≈ 5 x 10−8M⊙yr−1) and highly collimated (lenght to width ratio > 14), which explains why it has escaped previous detections with smaller telescopes. The high-velocity molecular gas is hot and optically thin, making the J = 2 — 1 line of 12CO ≈ 3-4 times stronger than the J = 1 — 0 line. Unexpectedly, this outflow does not appears to be driven by any of the embedded near-IR sources known in this region previous deep VLA surveys of the cloud (André, Montmerle, and Feigelson, 1987; Stine et al., 1988; André et al., in prep.). The outflow exciting source is thus probably a very low-luminosity ((L < 0.1L⊙) young stellar object. Using the 30-m equipped with the MPIfIR bolometer, we have very recently found (March 1989) that this object is the strongest continuum point source of L1688 at 1.3 mm. By analogy with L1551-IRS5 and HL Tau, the radio properties of this source suggest that it possesses a weak, possibly collimated, ionized wind and a relatively massive, cold circumstellar disk (Mdisk ≈0.1M⊙).

Infrared line observations of low-luminosity outflow sources

view Abstract Citations (27) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared Line Observations of Low-Luminosity Outflow Sources Smith, Howard A. ; Fischer, J. ; Geballe, T. R. ; Schwartz, P. R. Abstract The authors have observed the low-luminosity young stellar objects B5 IRS 1, L 1262, L 1489, L 1536, L 1551 IRS 5, and L 1582 in the infrared recombination lines of Brα and/or Brγ, and in the 5 GHz continuum. Line emission was detected from B5 IRS 1 in Brα, and from L 1536 in Brα and Brγ. Limits to the line flux from the other sources are derived. All have radio continuum fluxes <0.5 mJy at 6 cm. New upper limits of 1×10-20W cm-2 are set to the Brα flux from the double radio source in L 1455, and from NGC 2071 IRS 1, 2, and 3. The results indicate that low-luminosity outflow sources have strong ionized winds, despite the fact that their stellar ultraviolet production is relatively modest. The data also provide an indication that winds from low-luminosity objects possess a wide range of outflow parameters. A quantitative analysis of the data in terms of various stellar wind models is presented. Publication: The Astrophysical Journal Pub Date: May 1987 DOI: 10.1086/165198 Bibcode: 1987ApJ...316..265S Keywords: Early Stars; Infrared Astronomy; Line Spectra; Stellar Spectra; Emission Spectra; Hydrogen Clouds; Mass Flow Rate; Star Formation; Stellar Mass Ejection; Stellar Winds; Astrophysics; INFRARED: SOURCES; INFRARED: SPECTRA; NEBULAE: H II REGIONS; STARS: EMISSION-LINE; STARS: FORMATION; STARS: WINDS full text sources ADS | data products SIMBAD (9) NED (1)

Line excess in low-luminosity young stellar objects

Infrared spectra of 13 low-luminosity young stellar objects (YSOs) indicate that a majority have excess hydrogen line emission. This is in contradiction to a simple model of excess recombination line emission due to ionization from the first excited level of hydrogen. Thermal neutral atomic emission models do not violate energy conservation but require a few percent of the total luminosity utilized in heating an extended emission region. Whether there is a similar line excess emission mechanism for low- and high-luminosity YSOs is still open to question. 21 references.

Studies of Infrared Line Excess in Low-Luminosity Young Stellar Objects

Studies of Infrared Line Excess in Low-Luminosity Young Stellar Objects