Chemistry and kinematics in low-mass star-forming regions

This project studies the earliest stages of star formation in different environments by observing deuterated molecules and by measuring the deuterium fraction (i.e. the ratio between the column density of the species containing deuterium, in particular N2D+ and DCO+, and the column density of the same species containing hydrogen, N2H+ and H13CO+). The deuterium fraction is known to increase in pre-stellar cores, just before the formation of a protostar, and deuterated molecules are used to trace the kinematics and properties of pre-stellar cores. Deuterated molecules are important diagnostic tools for understanding the physical/chemical structure and kinematics of dense and cold gas in molecular clouds, i.e. to study the first steps in the process of star formation. In the PhD project, I focus my studies on two nearby low-mass star-forming regions: the Ophiuchus molecular cloud, the nearest cluster forming region, and the more quiescent Taurus molecular cloud. The project is based on three papers in which dense core chemistry and kinematics, as well as the substructure around pre-stellar cores, are discussed. In the first chapter, I study the deuterium fractionation in the dense cores of the L1688 clump in the Ophiuchus molecular cloud, one of the closest (∼120 pc) sites of star formation. A high deuterium fraction is one of the key features of pre-stellar cores, the dense cores on the verge of star formation. I study how the deuterium fraction depends on various physical conditions, such as gas density, temperature, turbulence, and depletion of CO. I show that regions of the same molecular cloud experience different dynamical, thermal, and chemical histories with consequences for the current star formation effciency and characteristics of future stellar systems. In the second chapter, I study the kinematics of dense cores in the L1495 filament in the Taurus molecular cloud. Interstellar filaments are common structures in molecular clouds and play an important role in the star-forming process. I use the N2H+(1-0) and N2D+(2-1) lines to trace the gas of the core centres where CO and other molecules are depleted, the H13CO+(1-0) and DCO+(2-1) lines to trace the core envelopes to search for any connections between core-scale and cloud-scale kinematics, and C18O(1-0) to reveal the kinematics of the filament gas. Unlike the L1688 cores in Ophiuchus, the L1495 cores show very similar properties - subsonic line widths, centroid velocities, velocity gradients, and specific angular momenta. I found that at the level of the cloud-core transition, the core's envelope is spinning up. At small scales the core material is slowing down implying a loss of specific angular momentum. The cloud material stays unaffected by the presence of rotating cores and protostars. In the third chapter, I study the substructure around a prototypical pre-stellar core, L1544, which is one of the isolated cores in the Taurus molecular cloud. The interferometric observations used reveal the structures of ∼700 au scale. The core shows a strong asymmetry in the distribution of methanol around the core. This asymmetry might be produced by asymmetric UV irradiation. The project gives a prospective to future work on the evolution of dense cores. As a part of the project, a number of observational proposals were submitted to single dish sub-mm telescopes and interferometers, for many of them the data already have been collected. These projects will continue the study of the evolution of dense cores, including their chemistry and kinematics. I will study the chemistry of the dense cores in L1495 (Taurus), L1688 (Ophiuchus), and B5 (Perseus), as well as the kinematics in L1688 and B5. I will compare the deuterium fractions of ions (N2D+/N2H+, DCO+/H13CO+) and neutrals (NH2D/NH3), as well as that of core centres (N2D+/N2H+) and envelopes (DCO+/H13CO+). I will study the small-scale structure of the selected cores with interferometric observations of high density tracers (NOEMA observations of para-NH2D towards the B213-10 core in Taurus). These observational data will be used in tandem with chemical models to unveil the chemical evolution of dense cores on the verge of star formation. Comparing the results from the different sets of dense cores embedded in Taurus, Perseus, and Ophiuchus, will allow us to quantify the environmental effects on the dynamical and chemical evolution of dense cores and the related star formation rate

Trajectory Retrieval and Component Investigations of Southern Polar Stratosphere Based on High Resolution Spectroscopy of Totally Eclipsed Moon Surface

Abstract. In this paper we present the high resolution spectral observations of the fragment of lunar surface during the total lunar eclipse of December 10, 2011. The observations were carried out with the fiber-fed echelle spectrograph at 1.2-m telescope in Kourovka Astronomical observatory (Ural mountains, central Russia). The observed radiation is transferred by tangent trajectory through the southern polar stratosphere before the reflection from the Moon and spectra contain a number of absorption bands of atmospheric gases (O 2 , O 3 , O 4 , NO 2 , H 2 O). High resolution analysis of three O 2 bands and O 4 absorption effects is used to trace the effective trajectory of solar emission through the stratosphere and to detect the contribution of scattered light. Bands of other gases allow us to measure their abundances along the trajectory

Methanol Mapping in Cold Cores : Testing Model Predictions*

Chemical models predict that in cold cores gas-phase methanol is expected to be abundant at the outer edge of the CO depletion zone, where CO is actively adsorbed. CO adsorption correlates with volume density in cold cores, and, in nearby molecular clouds, catastrophic CO freeze-out happens at volume densities above 10(4) cm(-3). The methanol production rate is maximized there and its freeze-out rate does not overcome its production rate, while the molecules are shielded from UV destruction by gas and dust. Thus, in cold cores, methanol abundance should generally correlate with visual extinction, which depends on both volume and column density. In this work, we test the most basic model prediction that maximum methanol abundance is associated with a local A ( V ) similar to 4 mag in dense cores and constrain the model parameters with the observational data. With the IRAM 30 m antenna, we mapped the CH3OH (2-1) and (3-2) transitions toward seven dense cores in the L1495 filament in Taurus to measure the methanol abundance. We use the Herschel/SPIRE maps to estimate visual extinction, and the (CO)-O-18(2-1) maps from Tafalla & Hacar to estimate CO depletion. We explored the observed and modeled correlations between the methanol abundances, CO depletion, and visual extinction, varying the key model parameters. The modeling results show that hydrogen surface diffusion via tunneling is crucial to reproduce the observed methanol abundances, and the necessary reactive desorption efficiency matches the one deduced from laboratory experiments.Peer reviewe

Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18

We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey (GAS), we identify regions of high density and near-constant, almost-thermal, velocity dispersion. Eighteen coherent structures are revealed, twelve in L1688 and six in B18, each of which shows a sharp "transition to coherence" in velocity dispersion around its periphery. The identification of these structures provides a chance to study the coherent structures in molecular clouds statistically. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 Msun, generally smaller than previously known coherent cores identified by Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). We call these structures "droplets." We find that unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.Comment: Accepted by ApJ in April, 201

The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt

We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with $A_V \gtrsim 7$ mag visible from the northern hemisphere in emission from NH$_3$ and other key molecular tracers. This first release includes the data for four regions in Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH$_3$ emission to dust continuum emission from Herschel, and find that the two tracers correspond closely. NH$_3$ is present in over 60\% of lines-of-sight with $A_V \gtrsim 7$ mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH$_3$ is detected toward ~ 40\% of lines-of-sight with $A_V \gtrsim 7$ mag. Moreover, we find that the NH$_3$ emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH$_3$ column densities through forward modeling of the hyperfine structure of the NH$_3$ (1,1) and (2,2) lines. We show that the NH$_3$ velocity dispersion, ${\sigma}_v$, and gas kinetic temperature, $T_K$, vary systematically between the regions included in this release, with an increase in both the mean value and spread of ${\sigma}_v$ and $T_K$ with increasing star formation activity. The data presented in this paper are publicly available.Comment: 33 pages, 27 figures, accepted to ApJS. Datasets are publicly available: https://dataverse.harvard.edu/dataverse/GAS_DR

Trajectory Retrieval and Component Investigations of Southern Polar Stratosphere Based on High Resolution Spectroscopy of Totally Eclipsed Moon Surface

In this paper we present the high resolution spectral observations of the fragment of lunar surface during the total lunar eclipse of December 10, 2011. The observations were carried out with the fiber-fed echelle spectrograph at 1.2-m telescope in Kourovka Astronomical observatory (Ural mountains, central Russia). The observed radiation is transferred by tangent trajectory through the southern polar stratosphere before the reflection from the Moon and spectra contain a number of absorption bands of atmospheric gases (O2, O3, O4, NO2, H2O). High resolution analysis of three O2 bands and O4 absorption effects is used to trace the effective trajectory of solar emission through the stratosphere and to detect the contribution of scattered light. Bands of other gases allow us to measure their abundances along the trajectory.Comment: 13 pages, 9 figure

The Green Bank Ammonia Survey: A Virial Analysis of Gould Belt Clouds in Data Release 1

We perform a virial analysis of starless dense cores in three nearby star-forming regions : L1688 in Ophiuchus, NGC 1333 in Perseus, and B18 in Taurus. Our analysis takes advantage of comprehensive kinematic information for the dense gas in all of these regions made publicly available through the Green Bank Ammonia Survey Data Release 1, which used to estimate internal support against collapse. We combine this information with ancillary data used to estimate other important properties of the cores, including continuum data from the James Clerk Maxwell Telescope Gould Belt Survey for core identification, core masses, and core sizes. Additionally, we used \textit{Planck} and \textit{Herschel}-based column density maps for external cloud weight pressure, and Five College Radio Astronomy Observatory $^{13}$CO observations for external turbulent pressure. Our self-consistent analysis suggests that many dense cores in all three star-forming regions are not bound by gravity alone, but rather require additional pressure confinement to remain bound. Unlike a recent, similar study in Orion~A, we find that turbulent pressure represents a significant portion of the external pressure budget. Our broad conclusion emphasizing the importance of pressure confinement in dense core evolution, however, agrees with earlier work.Comment: 35 pages, 8 tables, and 14 figures consisting of 16 .pdf files. Accepted for publication in the Astrophysical Journa

An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion

Spectral lines of ammonia, NH3, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH3 observations with the Very Large Array and Green Bank Telescope toward a prestellar core. These observations show an outer region with a fractional NH3 abundance of X(NH3) = (1.975 +/- 0.005) x 10(-8) (+/- 10% systematic), but it also reveals that, after all, the X(NH3) starts to decrease above a H-2 column density of approximate to 2.6 x 10(22) cm(-2). We derive a density model for the core and find that the break point in the fractional abundance occurs at the density n(H-2) similar to 2 x 10(5) cm(-3), and beyond this point the fractional abundance decreases with increasing density, following the power law n (-1.1). This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to n (-1) owing to the dominance of accretion in the central parts of starless cores.Peer reviewe