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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

THE IMPACT OF FREQUENCY STANDARDS ON COHERENCE IN VLBI AT THE HIGHEST FREQUENCIES

We have carried out full imaging simulation studies to explore the impact of frequency standards in millimeter and submillimeter very long baseline interferometry (VLBI), focusing on the coherence time and sensitivity. In particular, we compare the performance of the H-maser, traditionally used in VLBI, to that of ultra-stable cryocooled sapphire oscillators over a range of observing frequencies, weather conditions, and analysis strategies. Our simulations show that at the highest frequencies, the losses induced by H-maser instabilities are comparable to those from high-quality tropospheric conditions. We find significant benefits in replacing H-masers with cryocooled sapphire oscillator based frequency references in VLBI observations at frequencies above 175 GHz in sites which have the best weather conditions; at 350 GHz we estimate a 20%-40% increase in sensitivity over that obtained when the sites have H-masers, for coherence losses of 20%-10%, respectively. Maximum benefits are to be expected by using co-located Water Vapor Radiometers for atmospheric correction. In this case, we estimate a 60%-120% increase in sensitivity over the H-maser at 350 GHz.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

A PROPER MOTION STUDY OF THE HARO 6-10 OUTFLOW: EVIDENCE FOR A SUBARCSECOND BINARY

We present single-dish and very long baseline interferometry observations of an outburst of water maser emission from the young binary system Haro 6-10. Haro 6-10 lies in the Taurus molecular cloud and contains a visible T Tauri star with an infrared companion 1.''3 north. Using the Very Long Baseline Array, we obtained five observations spanning three months and derived absolute positions for 20 distinct maser spots. Three of the masers can be traced over three or more epochs, enabling us to extract absolute proper motions and tangential velocities. We deduce that the masers represent one side of a bipolar outflow that lies nearly in the plane of the sky with an opening angle of {approx}45 Degree-Sign . They are located within 50 mas of the southern component of the binary, the visible T Tauri star Haro 6-10S. The mean position angle on the sky of the maser proper motions ({approx}220 Degree-Sign ) suggests they are related to the previously observed giant Herbig-Haro (HH) flow which includes HH 410, HH 411, HH 412, and HH 184A-E. A previously observed HH jet and extended radio continuum emission (mean position angle of {approx}190 Degree-Sign ) must also originate in the vicinity of Haro 6-10S and represent a second, distinct outflow in this region. We propose that a yet unobserved companion within 150 mas of Haro 6-10S is responsible for the giant HH/maser outflow while the visible star is associated with the HH jet. Despite the presence of H{sub 2} emission in the spectrum of the northern component of the binary, Haro 6-10N, none of outflows/jets can be tied directly to this young stellar object.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

IRAS 18113-2503: THE WATER FOUNTAIN WITH THE FASTEST JET?

We present Expanded Very Large Array water maser observations at 22 GHz toward the source IRAS 18113-2503. Maser components span over a very high velocity range of {approx_equal} 500 km s{sup -1}, the second largest found in a Galactic maser, only surpassed by the high-mass star-forming region W49N. Maser components are grouped into a blueshifted and a redshifted cluster, separated by 0.''12. Further mid-IR and radio data suggest that IRAS 18113-2503 is a post-asymptotic giant branch star, thus a new bona fide member of the rare class of 'water fountains' (WFs). It is the evolved object with the largest total velocity spread in its water masers and with the highest velocity dispersion within its redshifted and blueshifted lobes ({approx_equal} 170 km s{sup -1}). The large total velocity range of emission probably indicates that IRAS 18113-2503 has the fastest jet among the known WF stars. On the other hand, the remarkably high velocity dispersion within each lobe may be interpreted in terms of shocks produced by an episode of mass ejection whose velocity increased up to very high values or, alternatively, by projection effects in a jet with a large opening angle and/or precessing motions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SEARCHING FOR NEW HYPERCOMPACT H II REGIONS

Hypercompact (HC) H II regions are, by nature, very young H II regions, associated with the earliest stages of massive star formation. They may represent the transition phase as an early B-type star grows into an O-type star. Unfortunately, so few HC H II regions are presently known that their general attributes and defining characteristics are based on small number statistics. A larger sample is needed for detailed studies and good statistics. Class II methanol masers are one of the best indicators of the early stages of massive star formation. Using the Arecibo Methanol Maser Galactic Plane Survey-the most sensitive blind survey for 6.7 GHz methanol masers to date-we selected 24 HC H II region candidates. We made Expanded Very Large Array continuum observations at 3.6 and 1.3 cm to search for HC H II regions associated with these masers. We identified six potential HC H II regions in our sample based on the presence of optically thick free-free emission. Overall, we find that 30% of the methanol masers have an associated centimeter radio continuum source (separation less than 0.1 pc), which is in general agreement with previous studies.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1971

Topics in atomic hydrogen standard research and applications

Hydrogen maser based frequency and time standards have been in continuous use at NASA tracking stations since February 1970, while laboratory work at Goddard has continued in the further development and improvement of hydrogen masers. Concurrently, experimental work has been in progress with a new frequency standard based upon the hydrogen atom using the molecular beam magnetic resonance method. Much of the hydrogen maser technology is directly applicable to the new hydrogen beam standard, and calculations based upon realistic data indicate that the accuracy potential of the hydrogen atomic beam exceeds that of either the cesium beam tube or the hydrogen maser, possibly by several orders of magnitude. In addition, with successful development, the hydrogen beam standard will have several other performance advantages over other devices, particularly exceptional stability and long continuous operating life. Experimental work with a new laboratory hydrogen beam device has recently resulted in the first resonance transition curves, measurements of relative state populations, beam intensities, etc. The most important aspects of both the hydrogen maser and the hydrogen beam work are covered.

Peters, H. E.↗
NASA NTRS1974

Rubidium 87 gas cell studies, phase 2

The design, development, and characteristics of a rubidium 87 maser are discussed. The design of a receiver capable of locking a crystal oscillator to the maser signal is reported. The subjects considered are: (1) maser construction, (2) maser control electronics, (3) the characteristics of the receiver, and (4) results of experimental maser tests.

Vanier, J.↗
NASA NTRS1978

Multistation refrigeration system

A closed cycle refrigeration (CCR) system is disclosed for providing cooling at different parts of a maser. The CCR includes a first station for cooling the maser's parts, except the amplifier portion, to 4.5 K. The CCR further includes means with a 3.0 K station for cooling the maser's amplifier to 3.0 K and, thereby, increases the maser's gain and/or bandwith by a significant factor. The means which provide the 3.0 K cooling include a pressure regulator, heat exchangers, an expansion valve, and a vacuum pump, which coact to cause helium, provided from a compressor, to liquefy and thereafter expand so as to vaporize. The heat of vaporization for the helium is provided by the maser amplifier, which is thereby cooled to 3.0 K.

Wiebe, E. R.↗
NASA NTRS1979

Non-equilibrium processes in interstellar molecules

The types of nonequilibrium emission and absorption by interstellar molecules are summarized. The observed brightness emission temperatures of compact OH and H2O sources are discussed using the concept of maser amplification. A single thermodynamic approach was used in which masers and anti-masers are considered as heat engines for the theoretical interpretation of the cosmic maser and anti-maser phenomena. The requirements for different models of pumping are formulated and a classification is suggested for the mechanisms of pumping, according to the source and discharge of energy.

Strelnitskiy, V. S.↗
NASA NTRS1981

Recent progress in the NASA-Goddard Space Flight Center atomic hydrogen standards program

At NASA Goddard Space Flight Center and through associated contractors, a broad spectrum of work is being carried out to develop improved hydrogen maser frequency standards for field use, improved experimental hydrogen maser frequency standards, and improved frequency and time distribution and measurement systems for hydrogen maser use. Recent progress in the following areas is reported: results on the Nr masers built by the Applied Physics Laboratory of Johns Hopkins University, the development of a low cost hydrogen maser at Goddard Space Flight Center, and work on a low noise phase comparison system and digitally phase locked crystal oscillator called the distribution and measurement system.

Reinhardt, V. S.↗
NASA NTRS1986

Particle propagation, wave growth and energy dissipation in a flaring flux tube

Wave amplification by downgoing particles in a common flare model is investigated. The flare is assumed to occur at the top of a coronal magnetic flux loop, and results in the heating of plasma in the flaring region. The hot electrons propagate down the legs of the flux tube towards increasing magnetic field. It is simple to demonstrate that the velocity distributions which result in this model are unstable to both beam instabilities and cyclotron maser action. An explanation is presented for the propagation effects on the distribution, and the properties of the resulting amplified waves are explored, concentrating on cyclotron maser action, which has properties (emission in the z mode below the local gyrofrequency) quite different from maser action by other distributions considered in the context of solar flares. The z mode waves will be damped in the coronal plasma surrounding the flaring flux tube and lead to heating there. This process may be important in the overall energy budget of the flare. The downgoing maser is compared with the loss cone maser, which is more likely to produce observable bursts.

White, S. M.↗
DOE OSTI2011

HIGH VELOCITY PRECESSING JETS FROM THE WATER FOUNTAIN IRAS 18286-0959 REVEALED BY VERY LONG BASELINE ARRAY OBSERVATIONS

We report the results of multi-epoch Very Long Baseline Array observations of the 22.2 GHz H{sub 2}O maser emission associated with the 'water fountain' IRAS 18286-0959. We suggest that this object is the second example of a highly collimated bipolar precessing outflow traced by H{sub 2}O maser emission, the other is W 43A. The detected H{sub 2}O emission peaks are distributed over a velocity range from -50 km s{sup -1} to 150 km s{sup -1}. The spatial distribution of over 70% of the identified maser features is found to be highly collimated along a spiral jet (jet 1) extended southeast to northwest; the remaining features appear to trace another spiral jet (jet 2) with a different orientation. The two jets form a 'double-helix' pattern which lies across {approx}200 mas. The maser distribution is reasonably fit by a model consisting of two bipolar precessing jets. The three-dimensional velocities of jet 1 and jet 2 are derived to be 138 km s{sup -1} and 99 km s{sup -1}, respectively. The precession period of jet 1 is about 56 years. For jet 2, three possible models are tested and they give different values for the kinematic parameters. We propose that the appearance of two jets is the result of a single driving source with significant proper motion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

CLUSTERED STAR FORMATION AND OUTFLOWS IN AFGL 2591

We report on a detailed study of the water maser kinematics and radio continuum emission toward the most massive and young object in the star-forming region AFGL 2591. Our analysis shows at least two spatial scales of multiple star formation, one projected across 0.1 pc on the sky and another one at about 2000 AU from a ZAMS star of about 38 M{sub Sun }. This young stellar object drives a powerful jet- and wind-driven outflow system with the water masers associated to the outflow walls, previously detected as a limb-brightened cavity in the NIR band. At about 1300 AU to the north of this object a younger protostar drives two bow shocks, outlined by arc-like water maser emission, at 200 AU either side of the source. We have traced the velocity profile of the gas that expands along these arc-like maser structures and compared it with the jet-driven outflow model. This analysis suggests that the ambient medium around the northern protostar is swept up by a jet-driven shock (>66 km s{sup -1}) and perhaps a lower-velocity ({approx}10 km s{sup -1}) wind with an opening angle of about 20 Degree-Sign from the jet axis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE MEGAMASER COSMOLOGY PROJECT. IV. A DIRECT MEASUREMENT OF THE HUBBLE CONSTANT FROM UGC 3789

In Papers I and II from the Megamaser Cosmology Project, we reported initial observations of H{sub 2}O masers in an accretion disk of a supermassive black hole at the center of the galaxy UGC 3789, which gave an angular-diameter distance to the galaxy and an estimate of H{sub 0} with 16% uncertainty. We have since conducted more very long baseline interferometric observations of the spatial-velocity structure of these H{sub 2}O masers, as well as continued monitoring of its spectrum to better measure maser accelerations. These more extensive observations, combined with improved modeling of the masers in the accretion disk of the central supermassive black hole, confirm our previous results, but with significantly improved accuracy. We find H{sub 0} = 68.9 {+-} 7.1 km s{sup -1} Mpc{sup -1}; this estimate of H{sub 0} is independent of other methods and is accurate to {+-}10%, including sources of systematic error. This places UGC 3789 at an angular-diameter distance of 49.6 {+-} 5.1 Mpc, with a central supermassive black hole of (1.16 {+-} 0.12) Multiplication-Sign 10{sup 7} M{sub Sun }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE MEGAMASER COSMOLOGY PROJECT. V. AN ANGULAR-DIAMETER DISTANCE TO NGC 6264 AT 140 Mpc

We present the direct measurement of the Hubble constant, yielding the direct measurement of the angular-diameter distance to NGC 6264 using the H{sub 2}O megamaser technique. Our measurement is based on sensitive observations of the circumnuclear megamaser disk from four observations with the Very Long Baseline Array, the Green Bank Telescope (GBT), and the Effelsberg telescope. We also monitored the maser spectral profile for 2.3 years using the GBT to measure accelerations of maser lines by tracking their line-of-sight velocities as they change with time. The measured accelerations suggest that the systemic maser spots have a significantly wider radial distribution than in the archetypal megamaser in NGC 4258. We model the maser emission as arising from a circumnuclear disk with orbits dominated by the central black hole. The best fit of the data gives a Hubble constant of H{sub 0} = 68 {+-} 9 km s{sup -1} Mpc{sup -1}, which corresponds to an angular-diameter distance of 144 {+-} 19 Mpc. In addition, the fit also gives a mass of the central black hole of (3.09 {+-} 0.42) Multiplication-Sign 10{sup 7} M{sub Sun }. The result demonstrates the feasibility of measuring distances to galaxies located well into the Hubble flow by using circumnuclear megamaser disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TOWARD A NEW GEOMETRIC DISTANCE TO THE ACTIVE GALAXY NGC 4258. III. FINAL RESULTS AND THE HUBBLE CONSTANT

We report a new geometric maser distance estimate to the active galaxy NGC 4258. The data for the new model are maser line-of-sight (LOS) velocities and sky positions from 18 epochs of very long baseline interferometry observations, and LOS accelerations measured from a 10 yr monitoring program of the 22 GHz maser emission of NGC 4258. The new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is 7.60 {+-} 0.17 {+-} 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is H{sub 0} = 72.0 {+-} 3.0 km s{sup -1} Mpc{sup -1}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The parallax of W43: A massive star-forming complex near the galactic bar

We report trigonometric parallax measurements of masers in the massive star-forming complex W43 from Very Long Baseline Array observations as part of the Bar and Spiral Structure Legacy Survey. Based on measurements of three 12 GHz methanol maser sources (G029.86–00.04, G029.95–00.01, and G031.28+00.06) and one 22 GHz water maser source (G031.58+00.07) toward W43, we derived a distance of 5.49{sub −0.34}{sup +0.39} kpc to W43. By associating the masers with CO molecular clouds, and associating the clouds kinematically with CO longitude-velocity spiral features, we assign W43 to the Scutum spiral arm, close to the near end of the Galactic bar. The peculiar motion of W43 is about 20 km s{sup –1} toward the Galactic Center and is very likely induced by the gravitational attraction of the bar.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

O{sub 2} emission toward Orion H{sub 2} peak 1 and the role of FUV-illuminated C-shocks

Molecular oxygen (O{sub 2}) has been the target of ground-based and space-borne searches for decades. Of the thousands of lines of sight surveyed, only those toward Rho Ophiuchus and Orion H{sub 2} Peak 1 have yielded detections of any statistical significance. The detection of the O{sub 2} N{sub J} = 3{sub 3}–1{sub 2} and 5{sub 4}–3{sub 4} lines at 487.249 GHz and 773.840 GHz, respectively, toward Rho Ophiuchus has been attributed to a short-lived peak in the time-dependent, cold-cloud O{sub 2} abundance, while the detection of the O{sub 2} N{sub J} = 3{sub 3}–1{sub 2}, 5{sub 4}–3{sub 4} lines, plus the 7{sub 6}–5{sub 6} line at 1120.715 GHz, toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity (12 km s{sup −1}) non-dissociative C-type shocks, both of which are fully shielded from far-ultraviolet (FUV) radiation, plus a postshock region that is exposed to an FUV field. We report a re-interpretation of the Orion O{sub 2} detection based on new C-type shock models that fully incorporate the significant effects the presence of even a weak FUV field can have on the preshock gas, shock structure, and postshock chemistry. In particular, we show that a family of solutions exists, depending on the FUV intensity, that reproduces both the observed O{sub 2} intensities and O{sub 2} line ratios. The solution in closest agreement with the shock parameters inferred for H{sub 2} Peak 1 from other gas tracers assumes a 23 km s{sup −1} shock impacting gas with a preshock density of 8 × 10{sup 4} cm{sup −3} and G{sub o} = 1, substantially different from that inferred for the fully shielded shock case. As pointed out previously, the similarity between the LSR velocity of all three O{sub 2} lines (≈11 km s{sup −1}) and recently measured H{sub 2}O 5{sub 32}–4{sub 41} maser emission at 620.701 GHz toward H{sub 2} Peak 1 suggests that the O{sub 2} emission arises behind the same shocks responsible for the maser emission, though the O{sub 2} emission is almost certainly more extended than the localized high-density maser spots. Since maser emission arises along lines of sight of low-velocity gradient, indicating shock motion largely perpendicular to our line of sight, we note that this geometry can explain not only the narrow (≲3 km s{sup −1}) observed O{sub 2} line widths despite their excitation behind a shock but also why such O{sub 2} detections are rare.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

NEAR-INFRARED H{sub 2} AND CONTINUUM SURVEY OF EXTENDED GREEN OBJECTS. II. COMPLETE CENSUS FOR THE NORTHERN GALACTIC PLANE

We discuss 94 Extended Green Objects (EGOs) in the northern Galactic plane cataloged by Cyganowski et al., based on near-infrared narrow H{sub 2} (2.122 μm) and continuum observations from the United Kingdom Infrared Telescope. This data set is three times larger than the one in our previous study and is unbiased by preselection. As discussed in the previous paper, the morphologies of the 4.5 μm emission generally resemble those of the near-infrared continuum, but are different from those of the H{sub 2} emission. Of our sample, only 28% of EGOs with H{sub 2} emission show similar morphologies between 4.5 μm and H{sub 2} emission. These results suggest that the 4.5 μm emission mainly comes from scattered continuum from the embedded young stellar objects, and partially from H{sub 2} emission. About half of EGOs are associated with H{sub 2} outflows, if the H{sub 2} outflow incompleteness is considered. The H{sub 2} outflow detection rate for EGOs with K-band detections (61%) is significantly higher than for those without K-band detections (36%). This difference may be due to the fact that both H{sub 2} and K-band emissions are associated with outflows, i.e., H{sub 2} emission and K-band continuum are associated with shocks and outflow cavities, respectively. We also compared the correlation between the H{sub 2} outflows and Class I 44 GHz methanol masers from the literature. The methanol masers can be located upstream or downstream of the H{sub 2} outflows and some bright H{sub 2} spots or outflows are not associated with methanol masers, suggesting that methanol masers and H{sub 2} emission trace different excitation conditions.

79 ASTRONOMY AND ASTROPHYSICS↗