Hélène R. Dickel

Bio: Hélène R. Dickel is an academic researcher from University of Illinois at Urbana–Champaign. The author has contributed to research in topics: Molecular cloud & H II region. The author has an hindex of 14, co-authored 41 publications receiving 694 citations. Previous affiliations of Hélène R. Dickel include University of Amsterdam & ASTRON.

The Berkeley-Illinois-Maryland-association millimeter array

Abstract: We describe the characteristics of the BIMA millimeter wave array at Hat Creek, California. The array is an aperture synthesis instrument consisting of 9 6-meter diameter antennas which may be deployed in three differenet configurations, with spacings ranging from 7 meters up to 1.3 km. At an observing frequency of 100 GHz these configurations yield maps with angular resolutions of 5", 2", and 0.4", over a 2' field. Larger fields may be mapped by using multiple pointings. For all but the oldest telescopes, the surface accuracy is ≤ 30 microns rms, and the aperture efficiency is 77% at 100 GHz. Background emission from antenna losses and spillover is very low, about 5 K after subtraction of the cosmic Bv(2.7K). Each antenna contains a single dewar which accommodates up to four separate receivers. SIS mixers are cooled to 3.2 K with novel Gifford-McMahon cycle refrigerators. Both the upper and lower sidebands of the first local oscillator are received and separated, providing two bands extending from 70-900 MHz on each side of the first local oscillator. The correlation spectrometer covers a bandwidth of up to 800 MHz, and provides up to 2048 channels for each antenna pair. There are four independently tunable spectral windows (in each sideband), allowing simultaneous observations of several different spectral lines. The spectral resolution ranges from 6 kHz to 3 MHz. For a single 8 hour track in one configuration, the sensitivity is approximately 1 mJy/beam in the 800 MHz wide continuum. Measurements of atmospheric phase fluctuations as functions of both time and baseline have been made; these indicate that routine imaging at angular resolutions of less than 1" at 100 GHz is possible only if self-calibration or some other means of phase correction can be applied. Examples of a few recent results are included. We note that 30% of the observing time on the array is granted to visitors.

A BIMA Array Survey of Molecules in Comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4)

Abstract: We present an interferometric search for large molecules, including methanol (CH_3OH), methyl cyanide (CH_3CN), ethyl cyanide (CH_3CH_2CN), ethanol (CH_3CH_2OH), and methyl formate (CH_3OCHO), in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) array. We also searched for transitions of the simpler molecules CS, SiO, HNC, HN^(13)C, and ^(13)CO. We detected transitions of CH_3OH and CS around comet LINEAR and one transition of CH_3OH around comet NEAT within a synthesized beam of ~20". We calculated the total column density and production rate of each molecular species using the variable temperature and outflow velocity (VTOV) model recently described by Friedel et al. Considering the molecular production rate ratios with respect to water, comet T7 LINEAR is more similar to comet Hale-Bopp, while comet Q4 NEAT is more similar to comet Hyakutake. It is unclear, however, due to such a small sample size, whether there is a clear distinction between a Hale-Bopp and Hyakutake class of comet or whether comets have a continuous range of molecular production rate ratios.

Physical conditions in regions of star formation

Abstract: ▪ Abstract The physical conditions in molecular clouds control the nature and rate of star formation, with consequences for planet formation and galaxy evolution. The focus of this review is on the...

The Resolved Properties of Extragalactic Giant Molecular Clouds

Abstract: We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and mass-luminosity relations of GMCs in a variety of extragalactic systems. Although the data are heterogeneous, we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. The property that shows the largest variability is their resolved brightness temperature, although even that is similar to the average Galactic value in most sources. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of objects that exhibit a remarkably uniform set of properties from galaxy to galaxy.

The 12C/13C Isotope Gradient Derived from Millimeter Transitions of CN: The Case for Galactic Chemical Evolution

Abstract: New measurements of 12 C/ 13 C ratios in Galactic molecular clouds have been conducted using the N ¼ 1 ! 0 transition of the CN radical. This species is unique in that it has extensive hyperfine structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular clouds, and they include sources that lie in the range of 0.09‐16.41 kpc in distance from the Galactic center (DGC). The ratios derived from CN indicate a gradient with Galactic distance of 12 C/ 13 C ¼ 6:01DGC þ 12:28. This gradient agrees rather closely with those derived from measurements of CO and H2CO. The least-squares fit to all data points for the three molecules is 12 C/ 13 C ¼ 6:21DGC þ 18:71. CO, CN, and H2CO are synthesized from quite varied reactions, and any 13 C fractionation must follow different pathways for these three species. The relatively good agreement between the 12 C/ 13 C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the 12 C/ 13 C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system ( 12 C/ 13 C ¼ 89) and local interstellar medium values ( 12 C/ 13 C � 68) of this ratio may be a result of 13 C enrichment since the formation of the solar system, as predicted by recent models.

Unveiling the circumstellar envelope and disk: A subarcsecond survey of circumstellar structures

Abstract: We present the results of a λ = 2.7 mm continuum interferometric survey of 24 young stellar objects in 11 fields. The target objects range from deeply embedded class 0 sources to optical T Tauri sources. This is the first subarcsecond survey of the λ = 2.7 mm dust continuum emission from young, embedded stellar systems. These multiarray observations, utilizing the high dynamic u-v range of the BIMA array, fully sample spatial scales ranging from 04 to 60'', thus allowing the first consistent comparison of dust emission structures in a variety of systems. The images show a diversity of structure and complexity. The optically visible T Tauri stars (DG Tauri, HL Tauri, GG Tauri, and GM Aurigae) have continuum emission dominated by compact (≤1'') circumstellar disks. In the cases of HL Tauri and DG Tauri, the disks are resolved. The more embedded near-infrared sources (SVS 13 and L1551 IRS 5) have continuum emission that is extended and compact. The embedded sources (L1448 IRS 3, NGC 1333 IRAS 2, NGC 1333 IRAS 4, VLA 1623, and IRAS 16293-2422) have continuum emission dominated by the extended envelope, typically ≥85% of the emission at λ = 2.7 mm. In many of the deeply embedded systems, it is difficult to uniquely isolate the disk emission component from the envelope extending inward to AU-sized scales. Simple estimates of the circumstellar mass in the optical/infrared and embedded systems are in the ranges 0.01-0.08 M☉ and 0.04-2.88 M☉, respectively. All of the target embedded objects are in multiple systems with separations on scales of ~30'' or less. Based on the system separation, we place the objects in three categories: separate envelope (separation ≥6500 AU), common envelope (separation 150-3000 AU), and common disk (separation ≤100 AU). These three groups can be linked with fragmentation events during the star formation process: separate envelopes from prompt initial fragmentation and the separate collapse of a loosely condensed cloud, common envelopes from fragmentation of a moderately centrally condensed spherical system, and common disk from fragmentation of a high angular momentum circumstellar disk.

The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data

Abstract: The BIMA Survey of Nearby Galaxies is a systematic imaging study of the 3 mm CO J = 1-0 molecular emission within the centers and disks of 44 nearby spiral galaxies. The typical spatial resolution of the survey is 6'' or 360 pc at the average distance (12 Mpc) of the sample. The velocity resolution of the CO observations is 4 km s-1, though most maps are smoothed to 10 km s-1 resolution. For 33 galaxies, multifield observations ensured that a region 190'' (〈D〉 = 10 kpc) in diameter was imaged. For the remaining 11 galaxies, which had smaller optical diameters and were on average farther away, single-pointing observations imaged a 100'' diameter (〈D〉 = 11 kpc) region. The sample was not chosen based on CO or infrared brightness; instead, all spirals were included that met the selection criteria of v⊙ ≤ 2000 km s-1, δ ≥ -20°, i ≤ 70°, D25 < 70', and BT < 11.0. The detection rate was 41/44 sources or 93%; of the three nondetections, one (M81) is known to have CO emission at locations outside the survey field of view. Fully sampled single-dish CO data were incorporated into the maps for 24 galaxies; these single-dish data comprise the most extensive collection of fully sampled, two-dimensional single-dish CO maps of external galaxies to date. We also tabulate direct measurements of the global CO flux densities for these 24 sources. For the remaining 20 sources, we collected sensitive single-dish spectra in order to evaluate the large-scale flux recovery. We demonstrate that the measured ratios of flux density recovered are a function of the signal-to-noise of the interferometric data. We examine the degree of central peakedness of the molecular surface density distributions and show that the distributions exhibit their brightest CO emission within the central 6'' in only 20/44 or 45% of the sample. We show that all three Local Group spiral galaxies have CO morphologies that are represented in SONG, though the Milky Way CO luminosity is somewhat below the SONG average, and M31 and M33 are well below average. This survey provides a unique public database of integrated intensity maps, channel maps, spectra, and velocity fields of molecular emission in nearby galaxies. It also lays the groundwork for extragalactic surveys by more powerful future millimeter-wavelength interferometers like CARMA and ALMA.