Stephen L. Scott

Dynamically driven evolution of the interstellar medium in M51

Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage.

Interferometric observations of 1. 4 millimeter continuum sources

A set of 1.4-mm continuum measurements for a sample of 15 Galactic and extragalactic sources obtained with a millimeter-wave interferometer are presented. Maps at 3 arcsec resolution reveal compact dust emission regions in L1551 IRS 5, HH 7-11, NGC 7538 IRS 1, DR 21(OH) and Arp 220. In Arp 220 about 50 percent of the total 1.4-mm flux originates from a compact source less than 650 pc in radius centered on the NIR nucleus. The mass of interstellar matter in the central source of Arp 220 is approximately 3 X 10 to the 9th solar masses. Comparison of the 1.4 and 2.7-mm dust emission in 10 Galactic sources indicates a spectra index of 3.0 + or - 0.2 for the frequency dependence of the observed small-scale flux. 15 refs.

Disk and Envelope Structure in Class 0 Protostars. II. High Resolution Millimeter Mapping of the Serpens Sample

We present high-resolution CARMA 230 GHz continuum imaging of nine deeply embedded protostars in the Serpens Molecular Cloud, including six of the nine known Class 0 protostars in Serpens. This work is part of a program to characterize disk and envelope properties for a complete sample of Class 0 protostars in nearby low-mass star-forming regions. Here, we present CARMA maps and visibility amplitudes as a function of uv-distance for the Serpens sample. Observations are made in the B, C, D, and E antenna configurations, with B configuration observations utilizing the CARMA Paired Antenna Calibration System. Combining data from multiple configurations provides excellent uv-coverage (4-500 kλ), allowing us to trace spatial scales from 10^2 to 10^4 AU. We find evidence for compact disk components in all of the observed Class 0 protostars, suggesting that disks form at very early times (t < 0.2 Myr) in Serpens. We make a first estimate of disk masses using the flux at 50 kλ, where the contribution from the envelope should be negligible, assuming an unresolved disk. The resulting disk masses range from 0.04 M_☉ to 1.7 M_☉, with a mean of approximately 0.2 M_☉. Our high-resolution maps are also sensitive to binary or multiple sources with separations ≳ 250 AU, but significant evidence of multiplicity on scales <2000 AU is seen in only one source.

Molecular gas in the starburst nucleus of M82

High-resolution interferometric observations of the CO(1-0) emission from the central one arcmin region of the peculiar galaxy M82 are presented. The observations show that the molecular gas can be divided into two components: a high concentration in the central 700 pc x 200 pc and extended regions that may be shell-like and filamentary in structure. The properties of the interstellar medium in the nuclear region of M82 implied by these and other observations are discussed, and the implications of the observations for star formation in M82 and other galaxies are considered. 76 references.

Millimeter interferometry of the molecular gas in ARP 20

The Owens Valley Millimeter Wave Interferometer has been used to map the 2.6-mm CO emission in the ultraluminous infrared galaxy Arp 220. Approximately 70 percent of the CO emission from the galaxy originates from an unresolved region less than 4 arcsec x 6 arcsec in size (corresponding to 1500 pc diameter) centered on the near-infrared nucleus. The mass of gas within this region is 10 to the 10th solar mass, which is about 30 times greater than that in an equivalent area of the Galaxy. This concentration could result in efficient star formation via cloud-cloud collisions and provide a significant accretion flow onto a compact, central object. 24 references.

The Owens Valley Millimeter Array

The telescopes and signal processing systems of the Owens Valley Millimeter Array are considered, and improvements in the sensitivity and stability of the instrument are characterized. The instrument can be applied to map sources in the 85 to 115 GHz and 218 to 265 GHz bands with a resolution of about 1 arcsec in the higher frequency band. The operation of the array is fully automated. The current scientific programs for the array encompass high-resolution imaging of protoplanetary/protostellar disk structures, observations of molecular cloud complexes associated with spiral structure in nearby galaxies, and observations of molecular structures in the nuclei of spiral and luminous IRAS galaxies.

The molecular bar and star formation in the nucleus of NGC 6946

The bright, nearby galaxy NGC 6946 in the 2.6 mm line of CO has been observed with the Owens Valley Millimeter-Wave Interferometer, synthesizing a beam 7.6 arc sec by 5.4 arc sec. The molecular gas in the central 1.5 kpc of the galaxy is confined to a bar of extent of approximately 2700 pc by 550 pc, which is centered on the nucleus and connects two dust lanes in the innermost spiral arms. The molecular bar is the likely source of the star formation activity suggested by optical and infrared studies in the nucleus of NGC 6946. 34 references.

CARMA: a new heterogeneous millimeter-wave interferometer

A new Combined Array for Research in Millimeter-wave Astronomy (CARMA) interferometer is being assembled from the existing Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers and the new Sunyaev?Zeldovich Array (SZA) at Cedar Flat, a site at 2,200 m altitude in the Inyo Mountains east of OVRO. The array will consist of 23 antennas of three different diameters, 3.5, 6.1 and 10.4 m, and will support observations in the 1 cm, 3 mm and 1.3 mm bands. The fist-light correlator is a flexible FPGA based system that will process up to 8 GHz of bandwidth on the sky for two subarrays consisting of 8 and 15 elements. The array configurations will offer antenna spacings from 5 m to 1.9 km allowing unprecedented high resolution and wide field imaging at millimeter wavelengths. Radiometers observing the 22 GHz water vapor emission line will be used to measure and correct for the water vapor induced path delay along the line of sight for each telescope and thereby minimize the time lost to “bad seeing”. This university based facility will emphasize technology development and student training along with leading edge astronomical research in areas ranging from Sunyaev-Zeldovich effect galaxy cluster surveys to studying protoplanetary disks.

Possible protostar near HH 7-11

Millimeter wavelength, aperture synthesis observations of the bipolar outflow region associated with Herbig-Haro objects HH 7-11 obtained at Owens Valley are presented. The circumstellar continuum emission, the circumstellar molecular-line emission, and the high-velocity CO emission from the blue-shifted lobe of the HH 7-11 bipolar outflow are analyzed. It is observed that the emission is resolved into a binary source; one source coincides with the embedded near-IR source SVS 13 and the companion to the southwest has been detected only at 98 and 110 GHz. It is suggested that, due to the lack of near-IR luminosity, the companion may be a true protostar and that the high-velocity molecular gas is distributed in an elongated, expanding, and highly inhomogeneous shell which surrounds a cavity containing the HH objects. 36 references.

Interferometric CO-18 observations of DR 21(OH) and L1551 IRS 5 at lambda = 1. 4 millimeters

High-resolution (3 arcsec) aperture synthesis maps in the J = 2 to 1 transition of CO-18 have been obtained for the star formation regions DR 21(OH) and L1551 IRS 5. Two compact sources, separated by 7 arcsec, have been discovered at the centers of H2O maser activity associated with DR 21(OH). Mass estimates from the 1.4 mm continuum and CO-18 line emission are similar to the 110 solar masses required for a bound system with the observed 4 km/s radial velocity difference, suggesting that a massive multiple star system is forming. The CO-18 emission from L1551 IRS 5 is extended in an elongated structure of radius 700 AU and mass of 0.1 solar masses. 17 refs.