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K. M. Menten

Publications and source records attributed to K. M. Menten.

At least 541 records · Page 30Linked to original sources
arXiv2003

Discovery of a luminous water megamaser in the FRII radiogalaxy 3C403

We report the first detection of a water megamaser in a radio-loud galaxy, 3C403. This object has been observed as part of a small sample of FRIIs with evidence of nuclear obscuration. The isotropic luminosity of the maser is ~1200 L(sun). With a recessional velocity of cz ~ 17680 km/s it is the most distant water maser so far reported. The line arises from the densest interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are observed, likely bracketing the systemic velocity of the galaxy. These may either arise from the tangentially seen parts of a nuclear accretion disk or from dense warm gas interacting with the radio jets.

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arXiv2003

A Class of Interstellar OH Masers Associated with Protostellar Outflows

Using the Very Large Array, we have detected weak OH maser emission near the Turner-Welch protostellar source in the W3 OH region. Unlike typical interstellar OH masers, which are associated with ultra-compact HII regions, our measured positions and proper motions (from Very Long Baseline Interferometry) indicate that these OH masers are associated with a bipolar outflow traced by strong H2O masers. These OH masers may be part of a class of interstellar OH masers that are associated with very young stars which have yet to, or may never, create ultra-compact HII regions. This class of OH masers appears to form near the edges of very dense material (within which H2O masers form), where total densities drop precipitously and interstellar UV radiation is sufficient to dissociate the H2O molecules. Observations of this class of OH masers may be an important way to probe the distribution of this important molecule in interstellar shocks at arcsecond resolution or better.

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arXiv2003

The Position, Motion, and Mass of Sgr A*

We report progress on measuring the position of Sgr A* on infrared images, placing limits on the motion of the central star cluster relative to Sgr A*, and measuring the proper motionof Sgr A* itself. The position of Sgr A* has been determined to within 10 mas on infrared images. To this accuracy, the gravitational source (sensed by stellar orbits) and the radiative source (Sgr A*) are coincident. Proper motions of four stars measured both in the infrared and radio indicate that the central star cluster moves with Sgr A* to within 70 km/s. Finally, combining stellar orbital information with an upper limit of 8 km/s for the intrinsic proper motion of Sgr A* (perpendicular to the Galactic plane), we place a lower limit on the mass of Sgr A* of 4 x 10^5 Msun.

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arXiv2003

The Flaring H2O Megamaser and Compact Radio Source in Mrk 348

We report on single-dish monitoring and extremely high angular resolution observations of the flaring H2O megamaser in the Seyfert 2 galaxy Mrk 348. The H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is very broad, with a FWHM of 130 km/s, and has no detectable high velocity components within 1500 km/s on either side of the strong line. Monitoring observations made with the Effelsberg 100m telescope show that the maser varies significantly on timescales as short as one day and that the integrated line flux is loosely correlated with the continuum flux. VLBA observations indicate that the maser emission arises entirely from a region less than 0.25 pc in extent, located toward a continuum component thought to be associated with the receding jet. We also report on integrated flux monitoring with the VLA between 1.4 and 43 GHz, and VLBA continuum observations of the milliarcsecond scale jets at 1.7, 8, 15, and 22 GHz. These observations have allowed us to tentatively pinpoint the location of the core, and also show the ejection of a new jet component during the current radio "flare."

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arXiv2002

II. Accurate Positions and Proper Motions of Stellar SiO Masers near the Galactic Center

Menten, Reid, Eckart and Genzel (1997) accurately determined the position of Sgr A* on an infrared image, by aligning the infrared image with positions measured for SiO masers, associated with infrared-bright evolved stars, at radio wavelengths. We now report greatly improved radio positions and, for the first time, proper motions of many stellar SiO masers at the Galactic Center. These positions and motions, coupled with better infrared imaging, allow a much improved location of Sgr A* on infrared images. With current data, infrared stars can be placed in a reference frame tied to Sgr A*, to an accuracy of approximately 10 mas in position and 1 mas/yr in motion. The position of Sgr A* is, within uncertainties, consistent with stellar accelerations and the measured orbital focus of the star S2. The star S2 was observed within 16 mas (130 AU in projection) of Sgr A* on 2002 May 02. Finally, we find that the central stellar cluster moves with Sgr A* to within approximatly 70 km/s.

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arXiv2002

Detection of HCN Direct l-Type Transitions Probing Hot Molecular Gas in the Proto-Planetary Nebula CRL 618

We report the detection of direct l-type transitions towards the proto-planetary nebula CRL 618 during a study of vibrationally excited carbon chains. The J=8,10,11,12,13,14 DJ=0 transitions of HCN in its first excited bending mode v2=1 were detected in absorption against the continuum of the central HII region making use of the Effelsberg 100m telescope and the Very Large Array. Additionally, the J=9 direct l-type transition was detected in emission presumably indicating a weak maser. All lines are blueshifted with respect to the systemic velocity of CRL 618 indicating that the lines originate from a hot, expanding circumstellar envelope. The HCN column density along the line of sight in front of the continuum is 2E18 cm^-2.

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arXiv2002

Physical conditions in the Protoplanetary Nebula CRL 618 derived from observations of vibrationally excited HCCCN

We used the Effelsberg 100m and IRAM 30m telescopes to observe vibrationally excited cyanoacetylene (HCCCN) in several rotational transitions toward the proto-planetary nebula CRL618. Lines from 9 different vibrationally excited states with energies ranging up to 1600 K above ground were detected. The lines show P Cygni profiles indicating that the HCCCN emission originates from an expanding and accelerating molecular envelope. The HCCCN rotational temperature varies with velocity, peaks at 520 K, 3 km/s blue-shifted from the systemic velocity and decreases with higher blueshift of the gas. The column density of the absorbing HCCCN is 3-6 x 1E17 cm^2. We modeled spectra based on spherical models of the expanding envelope which provide an excellent fit to the observations, and discuss the implications of the models. Additionally, lines from 13C substituted cyanoacetylene were observed. They can be used to constrain the 12C/13C ratio in this source to 10+-2.

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arXiv2002

Closest Star Seen Orbiting the Supermassive Black Hole at the Centre of the Milky Way

Measurements of stellar velocities and variable X-ray emission near the centre of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are likely supermassive black holes, but have not yet excluded several alternative configurations. Here we report ten years of high resolution astrometric imaging that allow us to trace two thirds of the orbit of the star currently closest to the compact radio source and massive black hole candidate SgrA*. In particular, we have observed both peri- and apocentre passages. Our observations show that the star is on a bound, highly elliptical Keplerian orbit around SgrA*, with an orbital period of 15.2 years and a peri-centre distance of only 17 light hours. The orbital elements require an enclosed point mass of 3.7+-1.5x10^6 solar masses. The data exclude with high confidence that the central dark mass consists of a cluster of astrophysical objects or massive, degenerate fermions, and strongly constrain the central density structure.

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arXiv2002

86 GHz SiO maser survey of late-type stars in the Inner Galaxy. I. Observational data

We present 86 GHz (v = 1, J = 2 -1) SiO maser line observations with the IRAM 30-m telescope of a sample of 441 late-type stars in the Inner Galaxy (-4 degr < l < +30 degr). These stars were selected on basis of their infrared magnitudes and colours from the ISOGAL and MSX catalogues. SiO maser emission was detected in 271 sources, and their line-of-sight velocities indicate that the stars are located in the Inner Galaxy. These new detections double the number of line-of-sight velocities available from previous SiO and OH maser observations in the area covered by our survey and are, together with other samples of e.g. OH/IR stars, useful for kinematic studies of the central parts of the Galaxy.

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arXiv2002

Observations of extragalactic masers in bright IRAS sources

We report the first results of an ongoing survey at 22 GHz with the 100-m Effelsberg telescope to search for water maser emission in bright IRAS sources. We have detected water vapor emission in IC 342. The maser, associated with a star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8 Mpc). Our detection raises the detection rate among northern galaxies with IRAS point source fluxes S(100micron) > 50 Jy to 16%.

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arXiv2002

CH3OH and H2O masers in high-mass star-forming regions

We present a comparison of ClassII CH3OH and H2O masers at high spatial resolution in a sample of 29 massive star-forming regions. Absolute positions of both maser types are compared with mm dust continuum, cm continuum and mid-infrared sources. Spatial correlations of the different tracers and kinematic signatures are discussed.

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arXiv2002

The association between water kilomasers and compact radio sources in the starburst galaxy NGC2146

We report the detection of 22 GHz water vapor emission toward the starburst galaxy NGC2146, made using the Effelsberg 100-m telescope. Interferometric observations with the Very Large Array (VLA) show that a part of the emission originates from two prominent sites of star formation that are associated with compact radio continuum sources, likely ultra-compact HII regions. It is concluded that the emission arises from the most luminous and distant water `kilomasers' detected so far. Our data increase the number of water maser detections in northern galaxies (Dec > -30 deg) with 100 micron IRAS point source fluxes > 50 Jy to 18%.

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arXiv2002

Imaging low order CO emission from the z=4.12 QSO PSS 2322+1944

We present observations of CO 1-0 and CO 2-1 emission from the z = 4.12 QSO PSS 2322+1944 using the Very Large Array. The CO emission is extended on a spatial scale of 2''. This extension could reflect the double nature of the QSO as seen in the optical, or could be diffuse emission with a (redshift corrected) mean brightness temperature of 2.8 K for the CO 2-1 line. We find the CO excitation conditions are lower than in two other IR-luminous z > 4 QSOs, suggesting the presence of a significant contribution from cooler, lower density molecular gas (n(H_2) = 5x10^3 /cm^3), although such a conclusion is complicated by the possibility of differential gravitational magnification.

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arXiv2002

IRAS 05358+3543: Multiple outflows at the earliest stages of massive star formation

We present a high-angular-resolution molecular line and millimeter continuum study of the massive star formation site IRAS 05358+3543. The most remarkable feature is a highly collimated (collimation factor ~10) and massive (>10 M_sun) bipolar outflow of 1 pc length, which is part of a quadrupolar outflow system. The three observed molecular outflows forming the IRAS 05358+3543 outflow system resemble, in structure and collimation, those typical of low-mass star-forming regions. They might therefore, just like low-mass outflows, be explained by shock entrainment models of jets. We estimate a mass accretion rate of 10^{-4) M_sun/yr, sufficient to overcome the radiative pressure of the central object and to build up a massive star, lending further support to the hypothesis that massive star formation occurs similarly to low-mass star formation, only with higher accretion rates and energetics.

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arXiv2002

Water maser emission in IC342

The detection of 22 GHz water vapor emission from IC342 is reported, raising the detection rate among northern galaxies with 100 micron IRAS point source fluxes > 50 Jy to 16%. The maser, associated with a star forming region 10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 solar luminosity (D = 1.8 Mpc). If the time variability is intrinsic, the maser size is < 1.5 x 10^(16) cm (< 0.5 mas) which corresponds to a brightness temperature of > 10^(9) K. The linewidth, luminosity, and rapid variability are reminiscent of the 8 km/s super maser in Orion-KL. A velocity shift of 1 km/s within two weeks and subsequent rapid fading is explained in terms of a chance alignment of two dense molecular clouds. Observations at 22 GHz toward Maffei2 are also reported, yielding a 5 sigma upper limit of 25 mJy for a channel spacing of 1.05 km/s.

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arXiv2001

Massive molecular outflows

We present a mapping study of massive molecular outflows in 26 high-mass star-forming regions at 11'' spatial resolution. Bipolar morpholgy is found in 80% of the sources and the collimation is higher than previously thought. Additionally, we find that well known low-mass correlations continue up to the high-mass regime, and accretion rates are around 10^(-4) Msun/yr rising as high as 10^(-3) Msun/yr. A tight correlation between the outflow and the core mass is established, implying that the accretion rate is a rather linear function of the initial core mass. All the data suggest that massive outflows are qualitatively similar and just quantitatively enhanced compared to their low mass counterparts.

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arXiv2001

Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2

We present the results of the first high angular resolution observations of SiO maser emission towards the star forming region W51-IRS2 made with the Very Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser source. One of these water maser complexes appears to trace a bow shock whose opening angle is consistent with the opening angle observed in the distribution of SiO maser emission. A comparison of our water maser image with an image constructed from data acquired 19 years earlier clearly shows the persistence and motion of this bow shock. The proper motions correspond to an outflow velocity of 80 km/s, which is consistent with the data of 19 years ago (that spanned 2 years). We have discovered a two-armed linear structure in the SiO maser emission on scales of ~25 AU, and we find a velocity gradient on the order of 0.1 km/s/AU along the arms. We propose that the SiO maser source traces the limbs of an accelerating bipolar outflow close to an obscured protostar. We estimate that the outflow makes an angle of <20 degrees with respect to the plane of the sky. Our measurement of the acceleration is consistent with a reported drift in the line-of-sight velocity of the W51 SiO maser source.

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