Papers by T. K. Sridharan
Astronomy & Astrophysics, 2021
Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent ... more Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent evidence shows that flares of these masers are driven by MYSO accretion bursts. Thus, maser monitoring can be used to identify such bursts which are hard to discover otherwise. Infrared observations reveal burst-induced changes in the spectral energy distribution (first and foremost a luminosity increase), which provide valuable information on a very intense phase of high-mass star formation. Aims. In mid-January 2019, flaring of the 6.7 GHz CH3OH maser (hereafter maser) of the MYSO G358.93-0.03 (hereafter G358) was reported. The international maser community initiated an extensive observational campaign which revealed extraordinary maser activity and yielded the detection of numerous new masering transitions. Interferometric imaging with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array resolved the maser emitting core of the star forming region and proved the...
The Astrophysical Journal, 2018
We carried out survey observations of HC 3 N and HC 5 N in the 42-45 GHz band toward 17 high-mass... more We carried out survey observations of HC 3 N and HC 5 N in the 42-45 GHz band toward 17 high-mass starless cores (HMSCs) and 35 high-mass protostellar objects (HMPOs) with the Nobeyama 45 m radio telescope. We have detected HC 3 N from 15 HMSCs and 28 HMPOs, and HC 5 N from 5 HMSCs and 14 HMPOs, respectively. The average values of the column density of HC 3 N are found to be 5.7 0.7 10 12 ( ) and 1.03 0.12 10 13 ( ) cm -2 in HMSCs and HMPOs, respectively. The average values of the fractional abundance of HC 3 N are derived to be 6.6 0.8 10 11 ´-( ) and 3.6 0.5 10 11 ´-( ) in HMSCs and HMPOs, respectively. We find that the fractional abundance of HC 3 N decreases from HMSCs to HMPOs using the Kolmogorov-Smirnov test. On the other hand, its average value of the column density slightly increases from HMSCs to HMPOs. This may imply that HC 3 N is newly formed in dense gas in HMPO regions. We also investigate the relationship between the column density of HC 3 N in HMPOs and the luminosity-to-mass ratio (L/M), a physical evolutional indicator. The column density of HC 3 N tends to decrease with the increase of the L/M ratio, which suggests that HC 3 N is destroyed by the stellar activities.
Detection of CO from infrared excess clouds (IRECs)
Bulletin of the Astronomical Society of India
Mid-IR Spectral Survey of High Mass Protostellar Objects
Spectra with R=100 have been obtained between 8 and 13 mum of 11 high-mass protostellar objects (... more Spectra with R=100 have been obtained between 8 and 13 mum of 11 high-mass protostellar objects (HMPOs) using MIRSI on the IRTF. The HMPOs are members of the survey of Sridharan (2002 ApJ 566, 931) that we had first found to be bright and compact in N band (10.5 mum) and 24.8 mum imaging on the IRTF. The spectra fall in three groups: 1. Deep silicate (9.7 mum) absorption with strong emission at both 8 and 13 mum (5 HMPOs), 2. Weak silicate absorption with a weak peak at 8.5 mum, and stronger emission at 13 mum (2 HMPOs), and 3. Approximately flat emission at 8 mum, rising to longer wavelengths (4 HMPOs). Spectra of members of each group are presented. We also present simple three component models that fit the spectra and the 24.8 mum photometry. The model components are hot dust in emission, warm dust in emission, and cold dust in extinction. The hot and warm components give estimates of the amount of gas and dust near each HMPO. The cold component gives estimates of the overlying c...
The RRI 10.4m millimeter-wave telescope
Bulletin of The Astronomical Society of India, 1993
The 10.4m millimeter-wave telescope of the Raman Research Institute (RRI) has been in operation s... more The 10.4m millimeter-wave telescope of the Raman Research Institute (RRI) has been in operation since 1988. It can observe over the 3-mm atmospheric transmission window (80-115 GHz) where rotational transitions of many molecules including CO lie. Dual polarization cryogenic receivers developed at RRI are in use with the telescope. Filter bands and acousto-optic spectrometers are employed as back-ends for carrying
NIR And MIR Emission From The Central High-mass Protostar In IRAS19410
We present IRTF-MIRSI photometry and a grism spectrum, IRAC photometry, and high-resolution Gemin... more We present IRTF-MIRSI photometry and a grism spectrum, IRAC photometry, and high-resolution Gemini mid-IR images of IRAS 19410+2336 mm1, a member of a rich cluster of mm cores located in one of the candidate regions of high-mass star formation identified by Sridharan et al. (2002). The Gemini images show a bipolar-like pair of IR sources separated by 0.7" that straddle
Nature, 2005
The processes leading to the birth of low-mass stars such as our Sun have been well studied, but ... more The processes leading to the birth of low-mass stars such as our Sun have been well studied, but the formation of high-mass (over eight times the Sun's mass, M(o)) stars remains poorly understood. Recent studies suggest that high-mass stars may form through accretion of material from a circumstellar disk, in essentially the same way as low-mass stars form, rather than through the merging of several low-mass stars. There is as yet, however, no conclusive evidence. Here we report the presence of a flattened disk-like structure around a massive 15M(o) protostar in the Cepheus A region, based on observations of continuum emission from the dust and line emission from the molecular gas. The disk has a radius of about 330 astronomical units (Au) and a mass of 1 to 8 M(o). It is oriented perpendicular to, and spatially coincident with, the central embedded powerful bipolar radio jet, just as is the case with low-mass stars, from which we conclude that high-mass stars can form through ac...
The Astrophysical Journal, 2002
We describe a systematic program aimed at identifying and characterizing candidate high-mass prot... more We describe a systematic program aimed at identifying and characterizing candidate high-mass protostellar objects (HMPOs). Our candidate sample consists of 69 objects selected by criteria based on those established by Ramesh & Sridharan (1997) using far-infrared, radio-continuum and molecular line data. Infrared-Astronomical-Satellite (IRAS) and Midcourse-Space-Experiment (MSX) data were used to study the larger scale environments of the candidate sources and to determine their total luminosities and dust temperatures. To derive the physical and chemical properties of our target regions, we observed continuum and spectral line radiation at millimeter and radio wavelengths. We imaged the free-free and dust continuum emission at wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H 2 O and CH 3 OH maser emission and observed the CO J = 2 → 1 and several NH 3 lines toward all sources in our sample. Other molecular tracers were observed in a subsample. While dust continuum emission was detected in all sources, most of them show only weak or no emission at 3.6 cm. Where detected, the cm emission is frequently found to be offset from the mm emission, indicating that the free-free and dust emissions arise from different subsources possibly belonging to the same (proto)cluster. A comparison of the luminosities derived from the cm emission with bolometric luminosities calculated from the IRAS far-infrared fluxes shows that the cm emission very likely traces the most massive source, whereas the whole cluster contributes to the far-infrared luminosity. Estimates of the accretion luminosity indicate that a significant fraction of the bolometric luminosity is still due to accretion processes. The earliest stages of HMPO evolution we seek to identify are represented by dust cores without radio emission. Line wings due to outflow activity are nearly omnipresent in the CO observations, and the molecular line data indicate the presence of hot cores for several sources, where the abundances of various molecular species are elevated due to evaporation of icy grain mantles. Kinetic gas temperatures of 40 sources are derived from NH 3 (1,1) and (2,2) data, and we compare the results with the dust temperatures obtained from the IRAS data. Comparing the amount of dust, and hence the gas, associated with the HMPOs and with ultracompact Hii regions (UCHiis) we find that the two types of sources are clearly separated in mass-luminosity diagrams: for the same dust masses the UCHii regions have higher bolometric luminosities than HMPOs. We suggest that this is an evolutionary trend with the HMPOs being younger and reprocessing less (stellar) radiation in the IR than the more evolved UCHiis regions. These results indicate that a substantial fraction of our sample harbors HMPOs in a pre-UCHii region phase, the earliest known stage in the high-mass star formation process.
Spitzer IRAC and MIPS Observations toward High-mass Star Forming Regions
Spitzer IRAC and MIPS observations toward high-mass star forming regions are presented. With the ... more Spitzer IRAC and MIPS observations toward high-mass star forming regions are presented. With the photometry from the IRAC and 2MASS, we derive the magnitude-color and color-color diagrams to investigate evolutionary stages of the YSOs in the fields. Toward sources well resolved in MIPS 24mum and/or 70mum, we fit their SEDs combining the (sub)mm data and subsequently estimate their total luminosities. By comparing the IRAC images with the (sub)mm continuum emission and CO molecular outflows, we study the dynamical interaction of massive young stars with the surrounding environment.
The Astrophysical Journal, 2007
We present studies of a massive protocluster AFGL5142 in the J=2-1 transition of the CO isotopolo... more We present studies of a massive protocluster AFGL5142 in the J=2-1 transition of the CO isotopologues, SO, CH 3 OH and CH 3 CN lines, as well as continuum at 225 GHz and 8.4 GHz. The 225 GHz continuum emission reveals three prominent peaks MM-1, MM-2 and MM-3 with estimated circumstellar material of 3, 3, and 2 M ⊙ , respectively. MM-1 and MM-2 are associated with strong CH 3 CN emission with temperatures of 90 ± 20 and 250 ± 40 K, respectively, while both MM-1 and MM-3 are associated with faint continuum emission at 8.4 GHz. The heating implied by the temperature indicates that MM-1 and MM-2 cores contain embedded massive young stars. Additional dust continuum peaks MM-4 and MM-5 appear to be associated with H 2 O masers. With many continuum sources at cm and mm wavelengths, and those already identified in the infrared, this region is forming a cluster of stars. A total of 22 lines from 9 molecules are detected. The line strength varies remarkably in the region. The strong SO emission is found both in molecular outflows and cloud cores. CH 3 OH emission, on the contrary, is much weaker in molecular outflows, and is detected toward hot cores MM-1 and MM-2 only, but is absent in less massive and perhaps less evolved cores MM-3, MM-4 and MM-5. The modeling of the CH 3 CN spectra yields an abundance relative to H 2 of 1 × 10 -8 and 4 × 10 -8 for the MM-1 and MM-2 cores, respectively. With similar core mass, the higher temperature and CH 3 CN abundance in the MM-2 core suggest that it might be at a more evolved stage than the MM-1 core. The CO and SO emission reveals at least three molecular outflows originating from the center of the dust core. The outflows are well collimated, with terminal velocities up to 50 km s -1 from the cloud velocity. Outflow A coincides with the SiO jet identified previously by . The maximum velocity of both the CO and SiO outflow increases with the distance from the driving source. Since jet-like outflows and disk-mediated accretion process are physically connected, the well collimated outflows indicate that even in this cluster environment, accretion is responsible for the formation of individual stars in the cluster.
The Astrophysical Journal, 2004
We present high resolution, multiwavelength continuum and molecular line images of the massive st... more We present high resolution, multiwavelength continuum and molecular line images of the massive star-forming region IRAS 18317-0757. The global infrared through millimeter spectral energy distribution can be approximated by a two temperature model (25K and 63K) with a total luminosity of approximately log(L/L ⊙ ) = 5.2. Previous submillimeter imaging resolved this region into a cluster of five dust cores, one of which is associated with the ultracompact H II region G23.955+0.150, and another with a water maser. In our new 2.7mm continuum image obtained with BIMA, only the UCH II region is detected, with total flux and morphology in good agreement with the free-free emission in the VLA centimeterwave maps. For the other four objects, the non-detections at 2.7mm and in the MSX mid-infrared bands are consistent with cool dust emission with a temperature of 13-40K and a luminosity of 1000-40000 L ⊙ . By combining single-dish and interferometric data, we have identified over two dozen virialized C 18 O cores in this region which contain ≈ 40% of the total molecular gas mass present. While the overall extent of the C 18 O and dust emission is similar, their emission peaks do not correlate well in detail. At least 11 of the 123 infrared stars identified by 2MASS in this region are likely to be associated with the star-forming cluster. Two of these objects (both associated with UCH II) were previously identified as O stars via infrared spectroscopy. Most of the rest of the reddened stars have no obvious correlation with the C 18 O cores or the dust continuum sources. In summary, our observations indicate that considerable fragmentation of the molecular cloud has taken place during the time required for the UCH II region to form and for the O stars to become detectable at infrared wavelengths. Additional star formation appears to be ongoing on the periphery of the central region where up to four B-type (proto)stars have formed amongst a substantial number of C 18 O cores.
The Astrophysical Journal, 2007
Using the Submillimeter Array (SMA) we have imaged for the first time the 321.226 GHz, 1029 -936 ... more Using the Submillimeter Array (SMA) we have imaged for the first time the 321.226 GHz, 1029 -936 ortho-H2 O maser emission. This is also the first detection of this line in the Cepheus A high-mass star-forming region. The 22.235 GHz, 616 -523 water masers were also observed with the Very Large Array 43 days following the SMA observations. Three of the nine detected submillimeter maser spots are associated with the centimeter masers spatially as well as kinematically, while there are 36 22 GHz maser spots without corresponding submillimeter masers. In the HW2 source, both the 321 GHz and 22 GHz masers occur within the region of ∼ 1" which includes the disk-jet system, but the position angles of the roughly linear structures traced by the masers indicate that the 321 GHz masers are along the jet while the 22 GHz masers are perpendicular to it. We interpret the submillimeter masers in Cepheus A to be tracing significantly hotter regions (600∼2000 K) than the centimeter masers.
The Astrophysical Journal, 2004
SMA observations of the massive star-forming region IRAS 18089-1732 in the 1 mm and 850 µm band r... more SMA observations of the massive star-forming region IRAS 18089-1732 in the 1 mm and 850 µm band reveal outflow and disk signatures in different molecular lines. The SiO(5-4) data show a collimated outflow in the northern direction. In contrast, the HCOOCH 3 (20-19) line, which traces high-density gas, is confined to the very center of the region and shows a velocity gradient across the core. The HCOOCH 3 velocity gradient is not exactly perpendicular to the outflow axis but between an assumed disk plane and the outflow axis. We interpret these HCOOCH 3 features as originating from a rotating disk that is influenced by the outflow and infall. Based on the (sub-)mm continuum emission, the mass of the central core is estimated to be around 38 M ⊙ . The dynamical mass derived from the HCOOCH 3 data is 22 M ⊙ , of about the same order as the core mass. Thus, the mass of the protostar/disk/envelope system is dominated by its disk and envelope. The two frequency continuum data of the core indicate a low dust opacity index β ∼ 1.2 in the outer part, decreasing to β ∼ 0.5 on shorter spatial scales.
The Astrophysical Journal, 2005
Combining mid-infrared data from the SPITZER Space Telescope with cold gas and dust emission obse... more Combining mid-infrared data from the SPITZER Space Telescope with cold gas and dust emission observations from the Plateau de Bure Interferometer, we characterize the Infrared Dark Cloud IRDC 18223-3 at high spatial resolution. The millimeter continuum data reveal a massive ∼184 M ⊙ gas core with a projected size of ∼28000 AU that has no associated protostellar mid-infrared counterpart. However, the detection of 4.5 µm emission at the edge of the core indicates early outflow activity, which is supported by broad CO and CS spectral line-wing emission. Moreover, systematically increasing N 2 H + (1-0) line-width toward the mm core center can be interpreted as additional evidence for early star formation. Furthermore, the N 2 H + (1-0) line emission reveals a less massive secondary core which could be in an evolutionary stage prior to any star formation activity.
The Astrophysical Journal, 2005
We present a study of molecular outflows toward a sample of 69 luminous IRAS point sources. The s... more We present a study of molecular outflows toward a sample of 69 luminous IRAS point sources. The sample is associated with dense molecular gas and has far-infrared luminosities ranging from 10 2 to 10 5 L , indicating these objects as regions likely forming high-mass stars. Mapping in the CO J ¼ 2 1 line shows that molecular outflows are ubiquitous in these regions. Most of the outflows have masses of tens of M . The typical dynamical timescale of the flow, without correcting for inclination of the flow axis, is a few times 10 4 yr. The typical energy in the outflows is 10 46 ergs, comparable to the turbulent energy in the core. Nearly half of the outflows show spatially resolved bipolar lobes. This indicates that low-mass young stars that coexist in the region are not responsible for the bipolar outflows observed. It is the more massive stars that drive the outflow. The large detection rate of outflows in the region favors an accretion process in the formation of massive stars. The maximum mass-loss rate in the wind is about 10 À4 M yr À1 . If these outflows are driven via accretion, the accretion rate should be as high as a few times 10 À4 M yr À1 . We compare CO outflows with images at near-infrared wavelengths from the Two Micron All Sky Survey (2MASS) archive and find that some outflows are associated with extended emission in the K band, which may be partly due to vibrationally excited H 2 emission at 2.12 m.
Monthly Notices of the Royal Astronomical Society, 1997
We examine the problem of reliably identifying ultracompact (UC) H II regions from Galaxy-wide da... more We examine the problem of reliably identifying ultracompact (UC) H II regions from Galaxy-wide data bases. It is shown that there is significant contamination in the sample of potential UC H II regions, selected from the lRAS PSC using the twocolour criterion of Wood & Churchwell, due to cloud cores with lower mass stars. We identify additional criteria to reduce this contamination. First, we use the differences in the radio emission between the cores with embedded high-and lowmass stars to segregate them. Then, through a latitude analysis we further improve the reliability. Effectively, the total number of potential UC H II regions is brought down by a factop of '" 4. This reduction eases the birth rate problem for massive stars, and offers a more reliable list of young massive stars for galactic structure studies. In the process, we have also identified a group of objects which may contain high-mass equivalents of the class 0 objects and clusters of intermediate-mass stars.
The Astrophysical Journal, 2014
We present submillimeter spectral line and dust continuum polarization observations of a remarkab... more We present submillimeter spectral line and dust continuum polarization observations of a remarkable hot core and multiple outflows in the high-mass star-forming region W43-MM1 (G30.79 FIR 10), obtained using the Submillimeter Array (SMA). A temperature of ∼ 400 K is estimated for the hot-core using CH 3 CN (J=19-18) lines, with detections of 11 K-ladder components. The high temperature and the mass estimates for the outflows indicate high-mass star-formation. The continuum polarization pattern shows an ordered distribution, and its orientation over the main outflow appears aligned to the outflow. The derived magnetic field indicates slightly super-critical conditions. While the magnetic and outflow energies are comparable, the B-field orientation appears to have changed from parsec scales to ∼ 0.1 pc scales during the core/star-formation process.
Observations of the bright-rimmed molecular clouds near the Cepheus OB2 association
Journal of Astrophysics and Astronomy, 1994
Massive stars have significant influence on the evolution of the interstellar medium. Bright rims... more Massive stars have significant influence on the evolution of the interstellar medium. Bright rims, cometary morphology of clouds, as well as their motion are some examples of the influence of massive stars on nearby molecular clouds. The cometary clouds in the Gum-Vela region are very good examples. In an attempt to understand the kinematics of the clouds in such regions we have carried out CO line observations towards bright-rimmed clouds near the OB Association Cep OB2. The radial velocities of the clouds are consistent with an expansion of the system at ≈ 4kms−1 away from the dominant O6.5V star in the association, HD206267. We find the rocket mechanism to be the most likely cause for expansion as found for both the Gum-Vela and the Rosette globules. We conclude that such expanding motions are quite common in regions near massive stars and make a brief comparison of the Cepheus system with the Gum-Vela system.
Proc. 12th Int. Symp. …, 2001
Hot electron bolometer (HEB) mixer elements, based on Niobium Titanium Nitride (NbTiN) thin film ... more Hot electron bolometer (HEB) mixer elements, based on Niobium Titanium Nitride (NbTiN) thin film technology, have been fabricated on crystalline quartz substrates over a 20 nm thick AlN buffer layer. The film was patterned by optical lithography, yielding bolometer elements that measure about 1 µm long and between 2 and 12 µm wide. These mixer chips were mounted in a fixed-tuned waveguide mixer block, and tested in the 600 and 800 GHz frequency range. The 3-dB output bandwidth of these mixers was determined to be about 2.5 GHz and we measured a receiver noise temperature of 270 K at 630 GHz using an intermediate frequency of 1.5 GHz. The receiver has excellent amplitude stability and the noise temperature measurements are highly repeatable. An 800 GHz receiver incorporating one of these mixer chips has recently been installed at the Sub-Millimeter Telescope in Arizona for field test and for astronomical observations.
Astronomy & Astrophysics, 2007
Context. Hot molecular cores are an early manifestation of massive star formation where the molec... more Context. Hot molecular cores are an early manifestation of massive star formation where the molecular gas is heated to temperatures >100 K undergoing a complex chemistry. Aims. One wants to better understand the physical and chemical processes in this early evolutionary stage. Methods. We selected the prototypical hot molecular core G29.96-0.02 being located at the head of the associated ultracompact Hii region. The 862 µm submm continuum and spectral line data were obtained with the Submillimeter Array (SMA) at sub-arcsecond spatial resolution. Results. The SMA resolved the hot molecular core into six submm continuum sources with the finest spatial resolution of 0.36 ×0.25 (∼1800 AU) achieved so far. Four of them located within 7800 (AU) 2 comprise a proto-Trapezium system with estimated protostellar densities of 1.4×10 5 protostars/pc 3 . The plethora of ∼80 spectral lines allows us to study the molecular outflow(s), the core kinematics, the temperature structure of the region as well as chemical effects. The derived hot core temperatures are of the order 300 K. We find interesting chemical spatial differentiations, e.g., C 34 S is deficient toward the hot core and is enhanced at the UCHii/ hot core interface, which may be explained by temperature sensitive desorption from grains and following gas phase chemistry. The SiO(8-7) emission outlines likely two molecular outflows emanating from this hot core region. Emission from most other molecules peaks centrally on the hot core and is not dominated by any individual submm peak. Potential reasons for that are discussed. A few spectral lines that are associated with the main submm continuum source, show a velocity gradient perpendicular to the large-scale outflow. Since this velocity structure comprises three of the central protostellar sources, this is not a Keplerian disk. While the data are consistent with a gas core that may rotate and/or collapse, we cannot exclude the outflow(s) and/or nearby expanding UCHii region as possible alternative causes of this velocity pattern.
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Papers by T. K. Sridharan