Engineering topics

Shetty, Rahul

Publications and source records attributed to Shetty, Rahul.

3 records

DOE OSTI2016

Distribution and mass of diffuse and dense CO gas in the Milky Way

Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star-forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in {sup 13}CO and {sup 12}CO, and a low column density component detected in {sup 12}CO, but not in {sup 13}CO. The “diffuse” and “dense” components are identified using a combination of smoothing, masking, and erosion/dilation procedures, making use of three large-scale {sup 12}CO and {sup 13}CO surveys of the inner and outer Milky Way. The diffuse component, which globally represents 25% (1.5 × 10{sup 8}M{sub ⊙}) of the total molecular gas mass (6.5 × 10{sup 8} M{sub ⊙}), is more extended perpendicular to the Galactic plane. The fraction of diffuse gas increases from ∼10%–20% at a galactocentric radius of 3–4 kpc to 50% at 15 kpc, and increases with decreasing surface density. In the inner Galaxy, a yet denser component traced by CS emission represents 14% of the total molecular gas mass traced by {sup 12}CO emission. Only 14% of the molecular gas mass traced by {sup 12}CO emission is identified as part of molecular clouds in {sup 13}CO surveys by cloud identification algorithms. This study indicates that CO emission not only traces star-forming clouds, but also a significant diffuse molecular ISM component.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

The CO-to-H{sub 2} conversion factor across the Perseus molecular cloud

We derive the CO-to-H{sub 2} conversion factor, X {sub CO} = N(H{sub 2})/I {sub CO}, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H{sub 2}) data with the I {sub CO} data from the COMPLETE Survey. We estimate an average X {sub CO} ∼ 3 × 10{sup 19} cm{sup –2} K{sup –1} km{sup –1} s and find a factor of ∼3 variations in X {sub CO} between the five sub-regions in Perseus. Within the individual regions, X {sub CO} varies by a factor of ∼100, suggesting that X {sub CO} strongly depends on local conditions in the interstellar medium. We find that X {sub CO} sharply decreases at A{sub V} ≲ 3 mag but gradually increases at A{sub V} ≳ 3 mag, with the transition occurring at A{sub V} where I {sub CO} becomes optically thick. We compare the N(H I), N(H{sub 2}), I {sub CO}, and X {sub CO} distributions with two models of the formation of molecular gas, a one-dimensional photodissociation region (PDR) model and a three-dimensional magnetohydrodynamic (MHD) model, tracking both the dynamical and chemical evolution of gas. The PDR model based on the steady state and equilibrium chemistry reproduces our data very well but requires a diffuse halo to match the observed N(H I) and I {sub CO} distributions. The MHD model matches our data reasonably well, suggesting that time-dependent effects on H{sub 2} and CO formation are insignificant for an evolved molecular cloud like Perseus. However, we find interesting discrepancies, including a broader range of N(H I), likely underestimated I {sub CO}, and a large scatter of I {sub CO} at small A{sub V} . These discrepancies most likely result from strong compressions and rarefactions and density fluctuations in the MHD model.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2013

QUANTIFYING OBSERVATIONAL PROJECTION EFFECTS USING MOLECULAR CLOUD SIMULATIONS

The physical properties of molecular clouds are often measured using spectral-line observations, which provide the only probes of the clouds' velocity structure. It is hard, though, to assess whether and to what extent intensity features in position-position-velocity (PPV) space correspond to 'real' density structures in position-position-position (PPP) space. In this paper, we create synthetic molecular cloud spectral-line maps of simulated molecular clouds, and present a new technique for measuring the reality of individual PPV structures. Using a dendrogram algorithm, we identify hierarchical structures in both PPP and PPV space. Our procedure projects density structures identified in PPP space into corresponding intensity structures in PPV space and then measures the geometric overlap of the projected structures with structures identified from the synthetic observation. The fractional overlap between a PPP and PPV structure quantifies how well the synthetic observation recovers information about the three-dimensional structure. Applying this machinery to a set of synthetic observations of CO isotopes, we measure how well spectral-line measurements recover mass, size, velocity dispersion, and virial parameter for a simulated star-forming region. By disabling various steps of our analysis, we investigate how much opacity, chemistry, and gravity affect measurements of physical properties extracted from PPV cubes. For the simulations used here, which offer a decent, but not perfect, match to the properties of a star-forming region like Perseus, our results suggest that superposition induces a ∼40% uncertainty in masses, sizes, and velocity dispersions derived from {sup 13}CO (J = 1-0). As would be expected, superposition and confusion is worst in regions where the filling factor of emitting material is large. The virial parameter is most affected by superposition, such that estimates of the virial parameter derived from PPV and PPP information typically disagree by a factor of ∼2. This uncertainty makes it particularly difficult to judge whether gravitational or kinetic energy dominate a given region, since the majority of virial parameter measurements fall within a factor of two of the equipartition level α ∼ 2.

79 ASTRONOMY AND ASTROPHYSICS↗