Kevin B. Marvel

Distribution and Motion of the Water Masers near IRAS 05413–0104

We have used the NRAO Very Long Baseline Array to image water masers associated with the low-luminosity, young stellar object (YSO) IRAS 05413-0104 at four epochs over a period of 10 weeks. The maser images show the detail of a symmetric, jetlike structure about 300 mas in extent. The 23° ± 2° position angle of the maser spot distribution is in excellent agreement with measured position angles for the observed larger scale H2 and SiO emission distributions; radial velocities are in agreement with SiO measurements showing redshifted gas to the southwest and blueshifted gas to the northeast. We have detected proper motions of numerous maser spots averaging 30 ± 12 mas yr−1, implying space velocities of 64 ± 27 km s−1 for a source distance of 450 pc. Some masers are located within a projected distance of 40 AU of the origin of expansion, the assumed position of the central source, suggesting that jet formation and acceleration takes place within this radius of the YSO. We compute an inclination of the outflow system to the plane of the sky of 4°, based on the relative magnitude of the proper motions and radial velocities of the masers. VLBI observations of water masers in YSOs are clearly demonstrated to be interesting and competitive probes of the kinematics of the gas in YSO jets.

High resolution observations of SiO masers: comparing the spatial distribution at 43 and 86 GHz

We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC + 10011 and cCyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz ( v = 1,2 J = 1-0) and the 86 GHz ( v = 1,2 J = 2-1) SiO masers. All the transitions have been imaged except the v = 2 J = 2-1 in IRC + 10011. We report the first VLBI map of the v = 1 J = 2-1 28SiO maser in IRC + 10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, cCyg. In this paper we have focused on the study of the relative spatial distribution of the different obsd. lines. We have found that in some cases the observational results are not reproduced by the current theor. pumping models, either radiative or collisional. In particular, for IRC + 10011, the v = 1 J = 1-0 and J = 2-1 28SiO lines have different spatial distributions and emitting region sizes, the J = 2-1 emission being located in an outer region of the envelope. For cCyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between ro-vibrational transitions of two abundant mol. species, H2O and 28SiO, is a possible explanation for the discrepancies found between the observations and the theor. predictions. We have introduced this overlapping process in the calcns. of the excitation of the SiO mol. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied. [on SciFinder (R)]

Rotation in the Envelope of an Evolved Star: Observations of the SiO Masers Toward NML Cygni

We have discovered a new peak of emission in the v = 1, J = 1-0, SiO maser spectrum of the supergiant NML Cygni. This new peak of emission is blueshifted by ~18 km s-1 from the previously detected SiO masers around VLSR ≈ 0 km s-1. A double-peaked SiO maser profile makes NML Cyg unique among evolved stars. We present the first images of the SiO masers toward NML Cyg observed using the Very Long Baseline Array. These images span the entire spectrum of NML Cyg, including the new blueshifted peak. We find that the masers form an elliptical ring of emission approximately 33 × 27 mas with the major axis oriented at a position angle of ~141°. The orientation of the ring is consistent with the northwest-southeast elongation previously observed in both the OH and H2O maser shells. The elongation axis of the SiO maser shell also provides an axis of symmetry for the velocity structure of the masers. The observed velocity structure is interpreted as rotation of the SiO maser shell with a velocity Vm sin i ≈ 11 km s-1 relative to a systemic velocity of -6.6 km s-1.

Time-Resolved AU-Scale Jets Traced by Masers in the IRAS 4A/B Regions of NGC 1333

We present results of VLBI observations of the water masers associated with IRAS 4A and IRAS 4B in the NGC 1333 star-forming region taken in four epochs over a two month period. Both objects have been classified as extremely young sources, and each source is known to be a multiple system. Using the Very Long Baseline Array, we detected 35 masers in epoch I, 40 masers in epoch II, 35 in epoch III, and 24 in epoch IV. Only one identified source in each system is associated with these masers. These data are used to calculate proper motions for the masers and trace the jet outflows within 100 AU of IRAS 4A2 and IRAS 4BW. In IRAS 4A2 there are two groups of masers, one near the systemic cloud velocity and one redshifted. They expand linearly away from each other at velocities of 53 km s−1. In IRAS 4BW, masers are observed in two groups that are blueshifted and redshifted relative to the cloud velocity. They form complex linear structures with a thickness of 3 mas (1 AU at a distance of 320 pc) that expand linearly away from each other at velocities of 78 km s−1. Neither of the jet outflows traced by the maser groups align with the larger scale outflows. We suggest the presence of unresolved companions to both IRAS 4A2 and 4BW.

The three-dimensional kinematics of water masers around the semiregular variable RT Virginis.

We report observations of water masers around the semiregular variable RT Virginis, which have been made with the Very Long Baseline Array of the National Radio Astronomy Observatory at five epochs, each separated by 3 weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The three-dimensional maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of 8 km s-1. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but they were less significant than those found in other semiregular variables. Systematic radial velocity drifts of individual maser features were found with amplitudes of ≤2 km s-1 yr-1. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km s-1 yr-1, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

The Circumstellar Environment of Evolved Stars as Revealed by Studies of Circumstellar Water Masers

This dissertation presents the results of a multi-epoch very long baseline interferometric (VLBI) study of water masers located in the extended atmospheres of seven evolved stars. The research was performed using the Very Long Baseline Array and Very Large Array of the National Radio Astronomy Observatory. Water masers are found to exist in the atmospheres of evolved, oxygen-rich stars where a population inversion of the rotational transition at 22 GHz can be maintained by collisional pumping. The masers are identified as individual pockets or filaments of gas, which have good velocity coherence and may be imaged using radio interferometry. Stellar winds are initiated in these sources by dust formation and acceleration of the gas through momentum coupling. The typical wind speeds in the region of the water masers are 10 to 20 \kms. The water masers in several evolved stars (VY CMa, VX Sgr, S Per, U Her, IK Tau, RX Boo and NML Cyg) have been observed at three epochs and exhibit proper motions consistent with the assumed source distances and the measured outflow velocity in the water maser region. Estimates of the distance to the sources using statistical approximation are in agreement with the currently accepted distances to the stars. The following stars had reliable distances determined using proper motion data: S Per (2.3 ± 0.5 kpc), VY CMa (1.4 ± 0.2 kpc), VX Sgr (1.4 ± 0.3 kpc). An upper limit for the distance of NML Cyg was obtained ( 3.5 kpc). The remaining stars had too few maser detections (RX BOO, IK Tau) or were not strong enough at all epochs (U Her) to self-calibrate using the VLBA. A detailed kinematic model was used to describe the flow motions of the gas in the maser region. The regions are found to be complex and not well modeled by uniform radial outflow, radial outflow with rotation, or radial outflow with acceleration. The reasons for this are explored and include the probable presence of anisotropic velocity fields induced through non-uniform dust formation near the star.

Water Maser Kinematics in the Jet of OH 12.8-0.9

We present Very Long Baseline Array observations of the kinematics of the water masers associated with OH 12.8-0.9, the fourth member of the so-called water-fountain class of sources. We find that the masers occupy two distinct regions at the ends of a bipolar jetlike structure oriented north-south, with the blueshifted masers located to the north and the redshifted masers to the south. The masers are distributed along arclike structures 12-20 mas across that are oriented perpendicular to the separation axis with an angular separation of ~110 mas on the sky. Our multiepoch observations show the two maser arcs to be expanding away from each other along the axis of separation. The relative proper motions of the two maser regions is 2.7 mas yr-1 (~105 km s-1 at the assumed distance of 8 kpc). The measured radial velocity difference between the northern blueshifted masers and the southern redshifted masers is 48.4 km s-1. The radial velocity, when combined with the proper motion, yields a three-dimensional expansion velocity of 58 km s-1 and an inclination angle of 24° for the jet. By combining our radial velocities with historical values, we estimate the three-dimensional acceleration of the masers to be ~0.63 km s-1 yr-1 and a dynamical age for the collimated outflow of ~90 yr.

Phase correction at millimeter wavelengths using observations of water vapor at 22 GHz

We present results from phase correction efforts at the Owens Valley Radio Observatory millimeter array (OVRO). A brief description of the theory of phase correction is followed by a description of the water line monitors (WLMs) constructed and placed on each of the six antennas of the array. A summary of the current software in place is also included. We present examples of data corrected using this technique and the first image created using radiometric phase correction at OVRO. The phase correction system is undergoing further development and will soon be made available for general observing at the array. A brief discussion of application of the technique for future arrays (e.g. MMA, LSA, etc.) is included as a conclusion to this contribution.

VLBI Observations of the Water Masers near LkHα 234 and BD +40 4124

I present observations made with the National Radio Astronomy Observatorys Very Large Array and Very Long Baseline Array of the water masers near the candidate Herbig Ae/Be star V1318 Cygni S (V1318S) in the vicinity of the Herbig Ae/Be star BD +40 4124 and near the Herbig Ae/Be star LkHα 234. The water masers are not found to be associated with either of the Herbig Ae/Be stars dominant in the optical but with other embedded sources. The masers appear to form in the outflows from these sources, some of which have been detected in radio continuum by other authors. The water masers near BD +40 4124 are found to be associated with the star V1318S, itself a candidate Herbig Ae/Be star. The water masers imaged with very long baseline interferometry resolutions near LkHα 234 are found to be associated with the continuum sources LkHα 234–VLA 2 and LkHα 234–VLA 3B. I do not detect maser emission from LkHα 234–VLA1. It is likely that the water maser emission originally thought to be associated with Herbig Ae/Be stars is in fact hosted by embedded young stellar objects, is linked to their outflowing jets, and is not found in their disks.

Astrophysics: A stellar performance

Using a network of telescopes spread across the United States, astronomers have made a movie of an expanding shell of gas that sheds light on the intricate processes of how a star is born.