Caltech Submillimeter Observatory

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

Abstract The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium 1 , which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs 2 , in at least one case there is evidence for an extreme magneto-ionic local environment 3,4 and a compact persistent radio source 5 . Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately $${903}_{-111}^{+72}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mn>903</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>111</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>72</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies 2,6 , far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications.

Sagittarius A*, the 4 10 6 M black hole candidate at the Galactic center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio Observatory's Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924-292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3 mm VLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

<i>Context. <i/>The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H<sub>2<sub/> ratio through the H fractionation.<i>Aims. <i/>We want to constrain the ortho/para H<sub>2<sub/> ratio across the L183 prestellar core. This is required to correctly describe the deuteration amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H<sub>2<sub/>D<sup>+<sup/> abundance to the sole ortho-H<sub>2<sub/>D<sup>+<sup/> column density measurement.<i>Methods. <i/>To constrain this ortho/para H<sub>2<sub/> ratio and derive its profile, we make use of the N<sub>2<sub/>D<sup>+<sup/>/N<sub>2<sub/>H<sup>+<sup/> ratio and of the ortho-H<sub>2<sub/>D<sup>+<sup/> observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models.<i>Results. <i/>We estimate the H<sub>2<sub/>D<sup>+<sup/> ortho/para ratio in the L183 cloud, and constrain the H<sub>2<sub/> ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (<i>≈<i/>0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5-2 10<sup>5<sup/> years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained.<i>Conclusions. <i/>A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H<sub>2<sub/> ratio should also place useful constraints on the dynamical evolution of prestellar cores.

SHARC II is a background-limited 350 &mu;m and 450 &mu;m facility camera for the Caltech Submillimeter Observatory undergoing commissioning in 2002. The key component of SHARC II is a 12 &times; 32 array of doped silicon 'pop-up' bolometers developed at NASA/Goddard. Each 1 mm &times; 1 mm pixel is coated with a 400 &Omega;/square bismuth film and located &lambda;/4 above a reflective backshort to achieve >75% absorption efficiency. The pixels cover the focal plane with >90% filling factor. At 350 &mu;m, the SHARC II pixels are separated by 0.65 &lambda;/D. In contrast to the silicon bolometers in the predecessor of SHARC II, each doped thermistor occupies nearly the full area of the pixel, which lowers the 1/f knee of the detector noise to <0.03 Hz, under load, at the bath temperature of 0.36 K. The bolometers are AC-biased and read in 'total power' mode to take advantage of the improved stability. Each bolometer is biased through a custom ~130 M&Omega; CrSi load resistor at 7 K and read with a commercial JFET at 120 K. The JFETs and load resistors are integrated with the detectors into a single assembly to minimize microphonic noise. Electrical connection across the 0.36 K to 4 K and 4 K to 120 K temperature interfaces is accomplished with lithographed metal wires on dielectric substrates. In the best 25% of winter nights on Mauna Kea, SHARC II is expected to have an NEFD at 350 &mu;m of 1 Jy Hz^-1/2 or better. The new camera should be at least 4 times faster at detecting known point sources and 30 times faster at mapping large areas compared to the prior instrument.

The 692 GHz para ground-state line of D2H+ has been detected at the Caltech Submillimeter Observatory towards the pre-stellar core 16293E. The derived D2H+ abundance is comparable to that of H2D+, as determined by observations of the 372 GHz line of ortho-H2D+. This is an observational verification of recent theoretical predictions (Roberts, Herbst & Millar 2003), developed to explain the large deuteration ratios observed in cold, high-density regions of the interstellar medium associated with low mass pre-stellar cores and protostars. This detection confirms expectations that the multiply deuterated forms of H3+ were missing factors of earlier models. The inclusion of D2H+ and D3+ in the models leads to predictions of higher values of the D/H ratio in the gas phase.

We define the sample for the second Caltech-Jodrell Bank VLBI survey. This is a sample of 193 flat- or gigahertz-peaked-spectrum sources selected at 4850 MHz. This paper presents images of 91 sources with a resolution of ~1 mas, obtained using VLBI observations at 4992 MHz with a global array. The remaining images and the integrated radio spectra will be presented in a forthcoming paper by Henstock et al.

This paper reports the measurement of the Sunyaev-Zeldovich effect (SZE) in the nearby clusters A478, A2142 and A2256 at 32 GHz. These clusters are part of a complete X-ray flux limited sample, which includes the previously measured Coma cluster. We find surface baryonic mass densities for the hot IGM within the 7.35' FWHM Gaussian beam of the OVRO 5.5-m telescope projected on the cluster centers. This is used to place lower limits on the ratio of the SZE determined baryonic mass to the gravitational binding mass. When combined with available X-ray models, the data from these three clusters and Coma yields a sample average value for the Hubble constant of 54 +/- 14 km/s/Mpc.

We use extensive 350 um polarimetry and continuum maps obtained with Hertz and SHARC II along with HCN and HCO+ spectroscopic data to trace the orientation of the magnetic field in the Orion A star-forming region. Using the polarimetry data, we find that the direction of the projection of the magnetic field in the plane of the sky relative to the orientation of the integral-shaped filament varies considerably as one moves from north to south. While in IRAS 05327-0457 and OMC-3 MMS 1-6 the projection of the field is primarily perpendicular to the filament it becomes better aligned with it at OMC-3 MMS 8-9 and well aligned with it at OMC-2 FIR 6. The OMC-2 FIR 4 cloud, located between the last two, is a peculiar object where we find almost no polarization. The projected angle of the field is more complicated in OMC-1 where it exhibits smooth variations in its orientation across the face of this massive complex. By combining the polarimetry and spectroscopic data we were able to measure a set of average values for the inclination angle of the magnetic field relative to the line of sight. We find that the field is oriented quite close to the plane of the sky in most places. More precisely, the inclination of the magnetic field is ~73 deg. around OMC-3 MMS 6, ~74 deg. at OMC-3 MMS 8-9, ~80 deg. at OMC-2 FIR 4, ~65 deg. in the northeastern part of OMC-1, and ~49 deg. in the Bar. We also present polarimetry data for the OMC-4 region located some 13 arcminutes south of OMC-1.

The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H13CN, HNC, HNCO, CO, CH3OH, and H2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H13CN (4–3) and HNCO (160,16–150,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H13CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System12C/13C ratio. The low HCN/H13CN abundance ratio may be in part due to contamination by an SO2line blended with the H13CN (4–3) line. In addition, chemical models suggest that the HCN/H13CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H13CN ratio observed in a comet is not inconsistent with the solar system12C/13C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10−4. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H2CO, and CH3OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H2CO emission suggests production of this species from dust grains.

We present 350 μm polarization observations of four low-mass cores containing Class 0 protostars: L483, L1157, L1448-IRS2, and Serp-FIR1. This is the second paper in a larger survey aimed at testing magnetically regulated models for core-collapse. One key prediction of these models is that the mean magnetic field in a core should be aligned with the symmetry axis (minor axis) of the flattened young stellar object inner envelope (aka pseudodisk). Furthermore, the field should exhibit a pinched or hourglass-shaped morphology as gravity drags the field inward toward the central protostar. We combine our results for the four cores with results for three similar cores that were published in the first paper from our survey. An analysis of the 350 μm polarization data for the seven cores yields evidence of a positive correlation between mean field direction and pseudodisk symmetry axis. Our rough estimate for the probability of obtaining by pure chance a correlation as strong as the one we found is about 5%. In addition, we combine together data for multiple cores to create a source-averaged magnetic field map having improved signal-to-noise ratio, and this map shows good agreement between mean field direction and pseudodisk axis (they are within 15°). We also see hints of a magnetic pinch in the source-averaged map. We conclude that core-scale magnetic fields appear to be strong enough to guide gas infall, as predicted by the magnetically regulated models. Finally, we find evidence of a positive correlation between core magnetic field direction and bipolar outflow axis.

We present new measurements of the ground state fine-structure line of atomic carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral to irregular, interacting and merging types. In comparison with CO(1-0), the CI(1-0) intensity stays fairly comparable in the different environments, with an average value of the ratio of the line integrated areas in Kkm/s of CI(1-0)/CO(1-0) = 0.2 +/- 0.2. However, some variations can be found within galaxies, or between galaxies. Relative to CO lines, CI(1-0) is weaker in galactic nuclei, but stronger in disks, particularly outside star forming regions. Also, in NGC 891, the CI(1-0) emission follows the dust continuum at 1.3mm extremely well along the full length of the major axis where molecular gas is more abundant than atomic gas. Atomic carbon therefore appears to be a good tracer of molecular gas in external galaxies, possibly more reliable than CO. Atomic carbon can contribute significantly to the thermal budget of interstellar gas. Cooling due to C and CO amounts typically to 2 x 10^{-5} of the FIR continuum or 5% of the CII line. However, C and CO cooling reaches 30% of the gas total, in Ultra Luminous InfraRed Galaxies, where CII is abnormally faint. Together with CII/FIR, the emissivity ratio CI(1-0)/FIR can be used as a measure of the non-ionizing UV radiation field in galaxies.

A survey of CO(3-2) emission from optically visible oxygen-rich Mira variable stars within 500 pc of the sun was conducted. A molecular envelope was detected surrounding 36 of the 66 stars examined. Some of these stars have lower outflow velocities than any Miras previously detected in CO. The average terminal velocity of the ejected material was 7.0 km/sec, about half the value found in Miras selected by infrared criteria. None of the stars with spectral types earlier than M 5.5 were detected. The terminal velocity increases as the temperature of the stellar photosphere decreases, as would be expected for a radiation driven wind. Mass loss rates for the detected objects were calculated, and it was found that there is no correlation between the infrared color of a Mira variable, and its mass loss rate. The mass loss rate is correlated with the far infrared luminosity, although a few stars appear to have extensive dust envelopes without any detectable molecular wind. A power-law relationship is found to hold between the mass loss rate and the terminal velocity of the ejected material. This relationship indicates that the dust envelope should be optically thick in the near infrared and visible regions of the spectrum when the outflow velocity is &gt; 17 km/sec. At the low end of the range of outflow velocities seen, the dust drift velocity may be high enough to lead to the destruction of the grains via sputtering. Half of the stars which were detected were re-observed in the CO(4-3) transition. A comparison of the outflow velocities obtained from these observations with those obtained by other investigators at lower frequencies shows no evidence for gradual acceleration of the outer molecular envelope.

Prestellar cores are unique laboratories for studying the chemical and physical conditions preceding star formation. We observed the prestellar core L1544 in the fundamental transition of ortho-H_(2)D^(+)(1_(1,0)-1_(1,1)) at different positions over 100" and found a strong correlation between its abundance and the CO depletion factor. We also present a tentative detection of the fundamental transition of para-D_(2)H^+ (1_(1,0)-1_(0,1)) at the dust emission peak. Maps in N_(2)H^+, N_(2)D^+, HCO^+, and DCO^+ are used and interpreted with the aid of a spherically symmetric chemical model that predicts the column densities and abundances of these species as a function of radius. The correlation between the observed deuterium fractionation of H^(+)_3, N_(2)H^+, and HCO^+ and the observed integrated CO depletion factor across the core can be reproduced by this chemical model. In addition, a simpler model is used to study the H_(2)D^+ ortho-to-para ratio. We conclude that, in order to reproduce the observed ortho-H_(2)D^+ observations, the grain radius should be larger than 0.3 μm.&#13;\n

ABSTRACT The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180° rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.

Aims.We present the results of a multi-transition CO observational program conducted on a sample of AGB and post-AGB stars envelopes. We have collected maps and single pointing observations of these envelopes in 5 rotational transitions ranging from J = 1–0 to J = 6–5, including in particular new observations of the CO line at 691 GHz at the CSO. The use of such a set of mm and submm CO line on stellar envelopes is rare and limited to the work of some authors on IRC+10216.Methods.Using a model for the CO emission of an AGB circumstellar envelope, in combination with a standard LVG approach, we have conducted a systematic modelling analysis using the whole set of CO data collected for a sample of 12 sources. We simultaneously fit all five transitions, taking into account the spatial information provided by the maps.Results.We find mass-loss rates in the range 1 10-7 to 4 10/yr, and envelope temperatures ranging from 20 K to 1000 K at a radius of 1016 cm. There seem to be a general anti-correlation between mass loss rates and temperature, the high mass loss rate AGBs having low temperatures, and vice versa. We show that most AGB data can be fitted using a constant mass loss rate, at least within the calibration uncertainties associated with the data collected at different frequencies. For some cases though (e.g. CIT 6, R Hya, χ Cyg), a change in the mass loss rate history needs to be invoked to reconcile data at low- and high-J, a scenario already mentioned by several authors to explain observations of WX Psc.

We present new observational data on the small-scale structure of the Taurus molecular cloud 1 (TMC-1) in the regime of 0.02-0.04 pc and 0.04-0.6 M☉. Our analysis is based on high-resolution, high-S/N, observations of an 8' × 8' area centered on the "cyanopolyyne peak" in the southeastern part of the TMC-1 ridge. The observations were made in the CCS 22 and 45 GHz transitions using NASA's Deep Space Network 70 m and 34 m telescopes at the Goldstone facility. The CCS emission in this region originates in three narrow components centered on LSR velocities of ~5.7, 5.9, and 6.1 km s-1. These components each represent a separate cylindrical feature elongated along the ridge. Among the three velocity components we identified a total of 45 clumps with a typical CCS column density of ~a few × 1013 cm-2, an H2 density of ~a few × 104 cm-3, and a mass in the range of 0.04-0.6 M☉. The statistical properties of these small-scale clumps are compared with those of the larger "NH3 cores" in cold clouds and "CS cores" in the hotter Orion region. The CCS clumps in TMC-1 are found to conform to Larson's scaling laws (relating observed line width to clump size) derived from the larger cores down to the small-scale regime (0.02 pc and 0.04 M☉). These clumps represent a regime in which microturbulence is small, amounting to ~10% of the thermal pressure inside a clump. Of the 45 clumps, only five appear to be gravitationally unstable to collapse. All unbound clumps have masses less than 0.2 M☉, while bound clumps have masses in the range 0.15-0.6 M☉. The 6.1 km s-1 velocity feature contains all the gravitationally unstable clumps and is the most likely site for future star formation.

We report the astronomical discovery of the 110→101 rotational transition within the ν2 = 1 vibrationally excited state of water vapor (H2O). Using the 10.4 m telescope of the Caltech Submillimeter Observatory, we detect strong maser emission in this line, which has a frequency near 658 GHz, toward a diverse sample of oxygen-rich red giant and supergiant stars. In circumstellar envelopes these 658 GHz H2O masers appear to be as common as SiO masers and H2O masers in other transitions, while we fail to detect 658 GHz H2O emission toward the W49 N and W51 N star-forming regions. For all of the 11 stars detected, the luminosity in the 658 GHz H2O transition is comparable to or higher than the luminosity of any other known SiO or H2O maser line.

We describe the characteristics of the 350 km polarimeter Hertz learned from laboratory tests and recent observations at the Caltech Submillimeter Observatory. Hertz contains a pair of 32 element arrays with 18A pixel spacing and 20A resolution. The instrument has been improved since initial observations in 1994 and 1995 ; the detector noise is now below the sky background noise. In excellent weather conditions on Mauna Kea, the noise-equivalent ux density (NEFD) for the measurement of polarized ux is 34 Jy Hz~1@2. The subtraction of correlated sky noise accomplished by the two-array design is crucial for achieving this performance. A method for analysis of our polarization data in the presence of the correlated noise is described. The instrumental polarization of Hertz is less than 0.5% across the detector array. Systematic errors in the measurement of polarization are less than 0.2%. We present a 350 km polarization map of Sgr B2 with 140 detections at greater than 3 p signicance. For our current database of all 350 km polarization measurements, the median polarization is 1.1%.

We present narrow-bandwidth submillimeter-wave sky opacity measurements made from the South Pole between 1995 February 9 and November 17, a period that includes an entire Austral winter. These measurements were made with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) using a heterodyne receiver tuned to a band that includes the 492 GHz fine-structure line of neutral atomic carbon. The zenith optical depth was below 0.72 half the time during the Austral winter and spring, and it reached values as low as 0.34 on day 232. The stability was also remarkably good: the opacity remained below 1.0 for weeks at a time. The South Pole is therefore an excellent site for submillimeter astronomy throughout the Austral winter and spring. The functional relationship between 492 GHz opacity and measured precipitable water vapor shows that a significant fraction of the opacity is caused by atmospheric constituents other than water vapor, indicating the need for accurate, site-dependent atmospheric modeling when opacity measurements at lower frequencies are extrapolated into the submillimeter.