Ruisheng Peng

ALIGNMENT BETWEEN FLATTENED PROTOSTELLAR INFALL ENVELOPES AND AMBIENT MAGNETIC FIELDS

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.

PROBING THE MAGNETIC FIELD WITH MOLECULAR ION SPECTRA. II.

We present further observational evidence in support of our earlier proposal (Houde et al. 2000) for detecting the presence of the magnetic field in molecular clouds by comparing spectra of molecular ions with those of neutral molecules. The ion lines tend to be narrower and do not show the wings due to flows, when the magnetic field is sufficiently strong. We obtained spectra for the optically thin lines of the H13CN and H13CO+ species in a sample of ten molecular clouds and found the results to be in agreement with our previous observations of the main isotopic species, HCN and HCO+, made in OMC-1, OMC-2, OMC-3 and DR21OH, thus eliminating the possibility of optical depth effects playing a role in the ion line narrowing. HCS+ was also detected in four of these star forming regions. We also discuss previously published results by (Benson et al. 1998) of N2H+ detections in a large sample of dark clouds. We show that the similarity in line widths between ion and neutral species in their sample is consistent with the relatively small amount of turbulence and other flows observed in these clouds.

On the Measurement of the Magnitude and Orientation of the Magnetic Field in Molecular Clouds

We demonstrate that the combination of Zeeman, polarimetry, and ion-to-neutral molecular line width ratio measurements permits the determination of the magnitude and orientation of the magnetic field in the weakly ionized parts of molecular clouds. Zeeman measurements provide the strength of the magnetic field along the line of sight, polarimetry measurements give the field orientation in the plane of the sky, and the ion-to-neutral molecular line width ratio determines the angle between the magnetic field and the line of sight. We apply the technique to the M17 star-forming region using a HERTZ 350 μm polarimetry map and HCO+-to-HCN molecular line width ratios to provide the first three-dimensional view of the magnetic field in M17.

ROTATION STATE OF COMET 103P/HARTLEY 2 FROM RADIO SPECTROSCOPY AT 1 mm*

The nuclei of active comets emit molecules anisotropically from discrete vents. As the nucleus rotates, we expect to observe periodic variability in the molecular emission line profiles, which can be studied through millimeter/ submillimeter spectroscopy. Using this technique we investigated the HCN atmosphere of comet 103P/Hartley 2, the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. We detected short-term evolution of the spectral line profile, which was stimulated by the nucleus rotation, and which provides evidence for rapid deceleration and excitation of the rotation state. The measured rate of change in the rotation period is +1.00 ± 0.15 minutes day−1 and the period itself is 18.32 ± 0.03 hr, both applicable at the epoch of the EPOXI encounter. Surprisingly, the spin-down efficiency is lower by two orders of magnitude than the measurement in comet 9P/Tempel 1 and the best theoretical prediction. This secures rotational stability of the comet’s nucleus during the next few returns, although we anticipate a catastrophic disruption from spin-up as its ultimate fate.

A COMPREHENSIVE SURVEY OF HYDROGEN CHLORIDE IN THE GALAXY

We report new observations of the fundamental J = 1–0 transition of HCl (at 625.918 GHz) toward a sample of 27 galactic star-forming regions, molecular clouds, and evolved stars, carried out using the Caltech Submillimeter Observatory. Fourteen sources in the sample are also observed in the corresponding H^(37)Cl J = 1–0 transition (at 624.978 GHz). We have obtained clear detections in all but four of the targets, often in emission. Absorptions against bright background continuum sources are also seen in nine cases, usually involving a delicate balance between emission and absorption features. From RADEX modeling, we derive gas densities and HCl column densities for sources with HCl emission. HCl is found in a wide range of environments, with gas densities ranging from 10^5 to 10^7 cm^(−3). The HCl abundance relative to H_2 is in the range of (3–30) × 10^(−10). Comparing with the chlorine abundance in the solar neighborhood, this corresponds to a chlorine depletion factor of up to ~400, assuming that HCl accounts for one-third of the total chlorine in the gas phase. The [^(35)Cl]/[^(37)Cl] isotopic ratio is rather varied, from unity to ~5, mostly lower than the terrestrial value of 3.1. Such variation is highly localized, and could be generated by the nucleosynthesis in supernovae, which predicts a ^(37)Cl deficiency in most models. The lower ratios seen in W3IRS4 and W3IRS5 likely confine the progenitors of the supernovae to stars with relatively large mass (≳25 M_⊙) and high metallicity (Z ~ 0.02).

The Alignment of the Magnetic Field and Collimated Outflows in Star-forming Regions: The Case of NGC 2071

The magnetic field is believed to play a crucial role in the process of star formation. From the support it provides during the initial collapse of molecular clouds to the creation of strong collimated jets responsible for large mass losses, current theories predict its importance in many different stages during the formation of stars. Here we report on observational evidence which tests one aspect that can be inferred from these theories: the alignment between the local magnetic field and collimated bipolar outflows in such environments. There is good evidence of an alignment in the case of NGC 2071.

SIMULTANEOUS DETERMINATION OF THE COSMIC RAY IONIZATION RATE AND FRACTIONAL IONIZATION IN DR 21(OH)

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, ζH2, and the ionization fraction, χe, in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH^+ and HCO^+ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H3^+ abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H3^+. We apply this technique to the case of the star-forming region DR 21(OH), where we successfully detected the (J = 3 → 2) and (J = 4 → 3) rotational transitions of HCNH^+. We also determine the C and O isotopic ratios in this source to be ^12C/^13C = 63 ± 4 and ^16O/^18O = 318 ± 64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H3^+) and Xe directly from observations, and estimate ζH2 accordingly. Our results, ζH2 = 3.1 x 10^-18 s^-1 and χe = 3.2 x 10^-8, are consistent with recent determinations in other objects.

Dichotomy in the Dynamical Status of Massive Cores in Orion

To study the evolution of high-mass cores, we have searched for evidence of collapse motions in a large sample of starless cores in the Orion molecular cloud. We used the Caltech Submillimeter Observatory telescope to obtain spectra of the optically thin (H^(13)CO^+) and optically thick (HCO^+) high-density tracer molecules in 27 cores with masses >1 M⊙. The red-and blue-asymmetries seen in the line profiles of the optically thick line, with respect to the optically thin line indicate that 2/3 of these cores are not static. We detect evidence for infall (inward motions) in 9 cores and outward motions for 10 cores, suggesting a dichotomy in the kinematic state of the nonstatic cores in this sample. Our results provide an important observational constraint on the fraction of collapsing (inward motions) versus noncollapsing (reexpanding) cores for comparison with model simulations.

An upper limit on gas production from 3200 Phaethon

Abstract Asteroid 3200 Phaethon resembles a comet in some ways, including a highly-eccentric orbit ( e ∼ 0.89 ) and a strong associated meteor shower (the Geminids). Yet this object has never been observed to exhibit any cometary activity, i.e., gas production. We observed 3200 Phaethon with the Caltech Submillimeter Observatory on two occasions, once while it was near its closest approach to Earth as it neared perihelion, and another while it was further from Earth post-perihelion. Observations of the J = 2 → 1 and J = 3 → 2 rotational transitions of 12CO, typically strong lines in comets and indicative of gas production, yielded no detection. Upper limits on the 12CO production of 1.8 × 10 28 and 7.6 × 10 28 molecules s −1 for Phaethon were determined on these two occasions.

Discovery of Hydrogen Fluoride in the Cloverleaf Quasar at z = 2.56

We report the first detection of hydrogen fluoride (HF) toward a high-redshift quasar. Using the Caltech Submillimeter Observatory, we detect the HF J = 1-0 transition in absorption toward the Cloverleaf, a broad absorption line quasi-stellar object at z = 2.56. The detection is statistically significant at the ~6σ level. We estimate a lower limit of 4 × 10^(14) cm^(–2) for the HF column density and using a previous estimate of the hydrogen column density, we obtain a lower limit of 1.7 × 10^(–9) for the HF abundance. This value suggests that, assuming a Galactic N(HF)/N H ratio, HF accounts for at least ~10% of the fluorine in the gas phase along the line of sight to the Cloverleaf quasar. This observation corroborates the prediction that HF should be a good probe of the molecular gas at high redshift. Measurements of the HF abundance as a function of redshift are urgently needed to better constrain the fluorine nucleosynthesis mechanism(s).