Ray S. Furuya

Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun

Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars ∼10 times more massive than the Sun (∼10M[circdot]), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10M[circdot] exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass (≳8M[circdot]) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude that the gas is falling inwards towards a very young star of ∼20M[circdot], in line with theoretical predictions of non-spherical accretion.

Water Maser Survey toward Low-Mass Young Stellar Objects in the Northern Sky with the Nobeyama 45 Meter Telescope and the Very Large Array

We give a detailed presentation of all the data from a multiepoch 22 GHz H2O maser survey mainly toward low-mass young stellar objects (YSOs) using the Nobeyama 45 m telescope and the Very Large Array (VLA). Our major results are already published (our Paper I). The Nobeyama survey is the first complete H2O maser survey toward known Class 0 sources in the northern sky (δ > -35°) and is one of the most sensitive surveys ever performed. The survey was conducted from 1996 May through 1999 March over 32 periods. A total of 606 observations were carried out toward 173 YSOs, including 36 unknown luminosity sources, and toward 31 preprotostellar cores (PPSCs) in the Ophiuchus star-forming region. We detected 149 spectra toward 39 YSOs and zero spectra toward the 31 PPSCs. Subsequent to the Nobeyama survey, we performed a follow-up interferometric survey with the VLA in order to associate 19 maser sources detected by the 45 m telescope with individual YSOs. In this paper we compile properties of 142 YSOs together with their H2O maser activity. On the basis of this data set, we use properties of water maser emission as a probe of jet phenomena in low-mass stars.

Rotating Disks in High-Mass Young Stellar Objects

We report on the detection of four rotating massive disks in two regions of high-mass star formation. The disks are perpendicular to known bipolar outflows and turn out to be unstable but long-lived. We infer that accretion onto the embedded (proto)stars must proceed through the disks with rates of ~10-2 M☉ yr-1.

Class I and Class II methanol masers in high-mass star-forming regions

Context. Among the tracers of the earliest phases in the massive star formation process, methanol masers have gained increasing importance. The phenomenological distinction between Class I and II methanol masers is based on their spatial association with objects such as jets, cores, and ultracompact Hii regions, but is also believed to correspond to different pumping mechanisms: radiation for Class II masers, collisions for Class I masers. Aims. We surveyed a large sample of massive star-forming regions in Class I and II methanol masers. The sample consists of 296 sources, divided into two groups named high and low according to their [25–12] and [60–12] IRAS colours. Previous studies indicate that the two groups may contain similar sources in different evolutionary stages, with the high sources representing the more evolved stages. Therefore, the sample can be used to assess a sequence for the occurrence of Class I and II methanol masers during the evolution of a massive star-forming region. Methods. We observed the 6 GHz (Class II) CH3OH maser with the Effelsberg 100-m telescope, and the 44 GHz and 95 GHz (Class I) CH3OH masers with the Nobeyama 45-m telescope. Results. We detected 55 sources in the Class II line (39 high and 16 low, 12 new detections); 27 sources in the 44 GHz Class I line (19 high and 8 low, 17 new detections); 11 sources in the 95 GHz Class I line (eight high and three low, all except one are new detections). The detection rate of Class II masers decreases with the distance of the source (as expected), whereas that of Class I masers peaks at ∼5 kpc. This could be due to the Class I maser spots being spread over a region <1 pc, comparable to the telescope beam diameter at a distance of ∼5 kpc. We also find that the two Class I lines have similar spectral shapes at 44 GHz and 95 GHz, which confirms that they have the same origin. Conclusions. Our statistical analysis shows that the ratio between the detection rates of Class II and Class I methanol masers is basically the same in high and low sources. Therefore, both maser types seem to be equally associated with each evolutionary phase. In contrast, all maser species (including H2O) have about three times higher detection rates in high than in low sources. This could indicate that the phenomena from which all masers originate become progressively more active with time during the earliest evolutionary phases of a high-mass star-forming region.

VLBI study of maser kinematics in high-mass SFRs. I. G16.59-0.05

Aims. To study the high-mass star-forming process, we have started a large project to unveil the gas kinematics close to young stellar objects (YSOs) through the Very Long Baseline Interferometry (VLBI) of maser associations. By comparing the high spatial resolution maser data, tracing the inner kinematics of the (proto)stellar cocoon, with interferometric thermal data, tracing the largescale environment of the hot molecular core (HMC) harbouring the (proto)stars, we can investigate the nature and identify the sources of large-scale motions. The present paper focuses on the high-mass star-forming region G16.59‐0.05. Methods. Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G16.59‐0.05: H2O at 22.2 GHz (4 epochs), CH3OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In addition, we performed high-resolution ( 0: 00 1), high-sensitivity (< 0:1 mJy) VLA observations of the radio continuum emission from the star-forming region at 1.3 and 3.6 cm. Results. This is the first work to report accurate measurements of the relative proper motions of the 6.7 GHz CH3OH masers. The di erent spatial and 3-D velocity distribution clearly indicate that the 22 GHz water and 6.7 GHz methanol masers are tracing di erent kinematic environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and the regular pattern of observed proper motions suggest that these masers are tracing rotation around a central mass of about 35 M . The flattened spatial distribution of the 6.7 GHz masers, oriented NW‐SE, suggests that they can originate in a disk/toroid rotating around the massive YSO which drives the 12 CO (2‐ 1) outflow, oriented NE‐SW, observed on arcsec scale. The extended, radio continuum source observed close to the 6.7 GHz masers could be excited by a wide-angle wind emitted from the YSO associated with the methanol masers, and such a wind is proven to be su ciently energetic to drive the NE‐SW 12 CO (2‐1) outflow. The H2O masers distribute across a region o set about 0: 00 5 to the NW of the CH3OH masers, in the same area where emission of high-density molecular tracers, typical of HMCs, was detected. We postulate that a distinct YSO, possibly in an earlier evolutionary phase than that exciting the methanol masers, is responsible for the excitation of the water masers and the HMC molecular lines.

Water Maser Survey toward Low-Mass Young Stellar Objects in the Northern Sky: Observational Constraints on Maser Excitation Conditions

We present the results from a multiepoch H2O maser survey toward low-mass young stellar objects using the Nobeyama 45 m telescope and the Very Large Array. Our Nobeyama survey is the first complete H2O maser survey toward known Class 0 sources in the northern sky (δ > -35°). During the series of the monitoring observations, we detected the maser emission toward none of the 31 pre-protostellar cores, 15 of 30 Class 0, two of 32 Class I, and zero of nine Class II sources. From this, we conclude that Class 0 sources are favorable sites to harbor the masers: the detection rates are derived to be 39.7% for Class 0, 4.0% for Class I, and 0.0% for Class II sources taking time variation into account. In addition, we found that the H2O maser luminosities in low-mass stars are more closely related to the luminosities of 100 AU scale radio jets rather than the mechanical luminosities of large-scale CO outflows. This fact suggests that the masers are associated with the shocked regions that are impacted by neutral protostellar jets emanating from the central stars. The drastic decrease of the maser detection rate in Class I sources is likely to be caused by the dissipation of dense gas around the central objects. We base this on the fact that the radio jets are found to have similar luminosities in Class 0 and Class I. It seems difficult even for active protostellar jets to excite masers in the remaining tenuous gas around Class I sources.

G24.78+0.08: A cluster of high-mass (proto)stars

We present the results of high angular resolution observations at millimeter wavelengths of the high-mass star forming region G24.78+0.08, where a cluster of four young stellar objects is detected. We discuss evidence for these to be high-mass (proto)stars in dierent evolutionary phases. One of the sources is detected only in the continuum at 2 and 2.6 mm and we suggest it may represent a good candidate of a high-mass protostar.

First results from a VLBA proper motion survey of H

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 SMMI, 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 -1 ) 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. The line of sight velocities of several features decrease at a rate of 1 km s -1 month -1 and the sky-projected relative motion of two features appears to be accelerated (decelerated) at a rate of 10-15 km s -1 month -1 . We propose that the shocks harboring the maser emission are slowed down as they proceed through the dense material surrounding the YSO. 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 SMM 1 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.

_\mathsf{2}

We present a study of the natal core harboring the low-mass Class 0 protostar GF 9-2 in the filamentary dark cloud GF 9 using the Nobeyama 45 m and CSO 10.4 m telescopes and the OVRO millimeter array. GF 9-2 is unique in the sense that it shows H_2O masers, a clear signpost of protostar formation, but does not have a high-velocity large-scale outflow. These facts indicate that the GF 9-2 core is early enough after star formation that it still retains some information of initial conditions for collapse. Our 350 μm dust continuum image identified a protostellar envelope with an extent of ≃5400 AU in the center of the molecular core ≃0.08 pc in size. The envelope mass is estimated to be ≃0.6 M_⊙ from the 350 μm flux density, while the LTE mass of the core is ≃3 M_⊙ from molecular lines. We found that the core has a radial density profile of ρ(r) ∝ r^(-2) for the 0.003 ≾ r/pc ≾ 0.08 region and that the velocity width of the core gas increases inward, while the outermost region maintains a velocity dispersion of a few times the ambient sound speed. If we interpret the broadened velocity width as infall, the core collapse can be well described by an extension of the Larson-Penston solution for the period after formation of a central star. We derived the mass accretion rate of ≃3 × 10^(-5) M_⊙ yr^(-1) from infall velocity of ≃0.3 km s^(-1) at r ≃ 7000 AU. Furthermore, we found evidence that a protobinary is being formed at the core center. All of the results suggest that the GF 9-2 core has been undergoing gravitational collapse for ≾5000 yr since the protostar formation and that the unstable state initiated the collapse ≃2 × 10^5 yr (the free-fall time) ago.

O masers in low-mass YSOs: the Serpens core and RNO 15-FIR

Aims. We carried out sub-arcsecond resolution observations towards the high-mass star formation region G19.61−0.23, in both continuum and molecular line emission. While the centimeter continuum images, representing ultra compact HII regions, will be discussed in detail in a forthcoming paper, here we focus on the (sub)mm emission, devoting special attention to the hot molecular core (HMC). Methods. A set of multi wavelength continuum and molecular line emission data between 6 cm and 890 μm were obtained with the Very Large Array, Nobeyama Millimeter Array, Owens Valley Radio Observatory millimeter array, and Submillimeter Array (SMA). These data were analyzed in conjunction with previously published data. Results. Our SMA observations resolve the HMC into three cores whose masses are on the order of 10^1−10^3 M_⊙. No submm core exhibits detectable free-free emission in the centimeter regime, but appear to be associated with masers and thermal line emission from complex organic molecules. Towards the most massive core, SMA1, the CH_3CN (18_K−17_K) lines provide hints of rotation about the axis of a jet/outflow traced by H_2O maser and H^(13)CO^+(1−0) line emission. Inverse P-Cygni profiles of the ^(13)CO (3−2) and C^(18)O (3−2) lines seen towards SMA1 indicate that the central high-mass (proto)star(s) is (are) still gaining mass with an accretion rate ≥3 × 10^(−3) M_⊙ yr^(−1). Owing to the linear scales and high accretion rate, we hypothesize that we are observing an accretion flow towards a star cluster in the making, rather than towards a single massive star.