Mapping the large scale depletion in the IRDC G351.77-0.51

G. Sabatini1,2,*, A. Giannetti2, J. Brand2, S. Bovino3, S. Leurini4

1 Dipartimento di Fisica e Astronomia, Università di Bologna, ITALY; 2 Istituto di Radioastronomia and Italian ALMA Regional Centre - INAF, Bologna, ITALY; 3 Department of Astronomy, Universitad de Concepciòn, CHILE; 4 Osservatorio Astronomico di Cagliari, ITALY

● The maps of dust (Tdust

) and excitation (Tex

) temperatures are needed to extract the column densities of H

2 and C18O respectively;

● Sources hidden in the dust → All flux gets re-emitted by the dust producing a grey-body spectrum;

● Tdust

map obtained by fitting a grey-body to the 70-870 μm continuum fluxes on a pixel-by-pixel basis;

● We applied the relation defined by Giannetti et al. (2017a) that links the dust temperature and the C18O excitation temperature (i.e., T

exC18O =

1.45Tdust

− 13). We show the results in Fig. 3;

Data analysis

REFERENCES: [1] Giannetti A., Leurini S., Wyrowski F., Urquhart J., et a., 2017a, A&A, 603, A33; [2] Leurini S., Pillai T., Stanke T., et al., 2011, A&A, 533, A85; [3] Schuller F., et al., 2009, A&A, 504, 415; [4] Thomas H. S., Fuller G. A., 2008, A&A, 479, 751; [5] T. Grassi, S. Bovino, et al., 2014, MNRAS, 439:2386–2419.

Depletion modelling

* sabatini@ira.inaf.it

Introduction

Fig. 4: Panel (a): Depletion factor map; Panel (b): column density of C18O applyng equation (1) onto the intgreted emission line intensity; Panel (c): H

2 column density map following Schuller et al. (2009) ;

Panel (d): Opacity correction maps obtained assuming the canonical ratio C17O/C18O=4 (Thomas & Fuller 2008).

Fig. 3: Excitation temperature map of the C18O obtained by a pixel-by-pixel SED-fitting from the 70-870 μm continuum fluxes and assuming the T

ex-T

dust relation defined in Giannetti et al. (2017a)

Fig. 1: Top: Sketch of the theoretical evolutionary stages for HMS formation. Bottom: Variation of the L/M ratio with time and rough correspondence with the infrared and radio continuum properties of the source.

The source

● Mass: ~2000 Mo;● Distance: < 1 kpc (~ 7.8 kpc from

Galactic Centre);● The presense of numerous star-formation

regions, its filamentary structure and its proximity, make this source the perfect choice to study the variation of the depletion factor, even in areas where the gas is more diffuse.

Fig. 2: LABOCA map of the 870 μm dust continuum emission from IRDC G351.77-0.51. The white contours are from 5σ (0.425 Jy/beam) to 2 Jy/beam in steps of 5σ; the black contours are from 2 Jy/beam to 5 Jy/beam in steps of 10σ. Image from Leurini et al. (2011).

Conclusion & future prospects

(b)(a)

(c) (d)

● Large-scale depletion map generated from the ratio (f

D)

between the expected and observed CO column density (Fig. 4a);

● The expected CO column density was obtained from the H

2 (Fig. 4c) assuming

the canonical factor of 2.1x10-7;

● The observed column density of CO (Fig. 4b) was calculated from the 0th moment of the C18O(2-1) observations:

→ opacity correction (Fig. 4d);→ represent all the constants and the terms depending on T

ex ;

→ TMB

is the main beam temperature;

● Selected three regions along the pringipal ridge of cold gas (i.e., regions C1, C2 and F1 in Fig.2);

N (C 18O)=C τ f (T ex)∫T MB d ν

N (C18O)

C τ

f (T ex)

(1)

● Extracted the mean radial profiles of N(H2) and N(C18O);

● Reconstructed an axi-symmetrical model assuming an N(H2) profile as in Tafalla et al.

(2002) – see Fig.6 top - and compared theory and observation.

[C2]

[F1]

Abstract

High-mass stars (HMS): ● Dominate the energy budget, stir, heat and ionize the gas in their environment, driving

physics and chemistry on a wide range of scales and determining how galaxies appear to us;● Produce most of the heavy elements and inject them in the interstellar medium;● The initial conditions that lead to their formation, how long this process takes, and which is

their role in regulating the star-formation activity, are still open questions.● Nevertheless, in recent years, a large number of observations has allowed us to divide high-

mass star-forming regions into empirical evolutionary classes depending on their observed chemo-physical properties (e.g., Giannetti et al. 2017a).

● Although the objects can be arranged in an evolutionary sequence, as sketched in Fig. 1, it is not yet possible to assign a phase and accurately determine its duration.

● As the physical conditions change with time, so does the chemical composition of the environment;

● Chemo-dynamical models are able to derive the time-dependence of the relative abundances of different chemical species, thus making it possible to assign an age to each individual class.

Evidence of depletion of the carbon-monoxide and its isotopologues has been widely observed in sources. This has important consequences on the chemistry of the interstellar medium,

such as the enhancement of the fraction of deuterated molecules. In cold and dense environments (i.e., T<20 K and n > a few 10 4 cm-3) the species freeze onto the dust grains - the so called depletion process – with a significant impact on the formation of more complex molecules and the chemical properties of the environment. A key issue is the size of the so-called full depletion radius R

fd (i.e., the radius within which CO is completely locked onto dust grains).

We tackle this problem by studying the Infrared Dark Cloud (IRDC) G351.77-0.51 from the ATLASGAL-TOP100 sample (a representative sample of high-mass star forming regions in different evolutionary stages). Multi-wavelength continuum observations performed with the (ESA) Herschel Space Observatory and the LABOCA camera at the Atacama Pathfinder EXperiment (APEX), together with APEX C18O (2-1) line observations, allowed us to recover the large-scale depletion map of the source.

We built a simple model to investigate the depletion in the inner regions of clumps and of the filament. The model suggests that the depletion radius ranges between 5x10 3 and 104 AU by

changing the depletion degree from 10 to the full depletion condition for R<Rfd

and reaching number densities between 7x104 and 1.2x105 cm-3. This is crucial information to probe the different chemical processes inside/outside the full depletion area.

Fig. 6: Top: From right to left, we show the H2 column density – N(H

2) – the C18O column density

– N(C18O) – and the mean depletion radial profile – fD – as a function of distance from the

central collapse in the C2 region of Fig. 2. Bottom: The same for the F1 region in Fig. 2.

● The basic model assumption is that both profiles have radial symmetry with the radius starting from the central collapse;

● To simulate different depletion degrees, we changed the relative abundance C18O/H

2 to

Fig. 5: Model applied to simulate the depletion effect. In blue we show the number density profile of H

2; in green the step-

function reproducing the variation of the degree of depletion inside/outside R

fd. The

C180 profile is obtained by the convolution of the other two.

reproduce a theoretical depletion factor f

D∈(10, 102,

103, ∞) for R<Rfd;

● We smoothed the profiles to the resolution of the data to compare to observations;

● Radial averages;● Extraction of the

best-fit value for Rfd

as a function of fD.

The model suggests that the depletion radius ranges between 5x103 and 104 AU by changing the depletion degree from 10 to the full depletion condition (i.e. f

D=∞) for R<R

fd. At the distance of

Rfd, densities reach values of 7x104 and 1.2x105

cm-3. Even by using a simple model, if these quantities are confirmed in other sources, they will provide crucial information for the different chemical processes acting inside/outside the full depletion region. As a next step, we will construct a chemo-dynamical model of the collapse plus warm-up phase using KROME, one of the few open source packages able to treat dust chemistry and -physics (Grassi et al. 2014). We will compare the results with the relative abundances observed in the TOP100 sample.This is expected to help find the best chemical tracers for each evolutionary phase of the high-mass star formation process, and estimate their duration.

● Is the most massive, closest filament in the ATLASGAL survey ( see Fig. 2 );● Early evolutionary stage, lots of cold material and dark in the MIR;

.