We present results from radiative transfer modeling of the continuum emission towards Sagittarius B2 (hereafter Sgr B2). We have developed a radiative transfer framework – Pandora – that employs RADMC-3D (Dullemond 2012) for a self-consistent determination of the dust temperature. With this pipeline, we have set-up a single model that consistently reproduces the thermal dust and free-free continuum emission of Sgr B2 spanning four orders of magnitude in spatial scales (0.02–45 pc) and two orders of magnitude in frequency (20–4000 GHz)

Bergin, E.

Comito, C.

Csengeri, T.

Lis, D. C.

Molinari, S.

Möller, Th.

Qin, S. L.

Rolffs, R.

Schilke, P.

Schmiedeke, A.

Sánchez-Monge, Á.

A. Schmiedeke

P. Schilke

Th. Möller

Á. Sánchez-Monge

E. Bergin

C. Comito

T. Csengeri

D.C. Lis

S. Molinari

S.L. Qin

R. Rolffs

Crossref

EAS Publications Series

Continuum radiative transfer Modeling of Sagittarius B2

Möller, T.

Lis, D.

Qin, S.

Rolffs,

MPG.PuRe

Rolffs

Caltech Authors - Main

Conditions and Impact of Star Formation
R. Simon, R. Schaaf and J. Stutzki (eds)
EAS Publications Series, 75-76 (2015) 171–174
CONTINUUM RADIATIVE TRANSFER MODELING OF
SAGITTARIUS B2
A. Schmiedeke1, P. Schilke1, Th. Mo¨ller1, A´. Sa´nchez-Monge1,
E. Bergin2, C. Comito1, T. Csengeri3, D.C. Lis4,5, S. Molinari6,
S.L. Qin7 and R. Rolﬀs1
Abstract. We present results from radiative transfer modeling of the
continuum emission towards Sagittarius B2 (hereafter Sgr B2). We
have developed a radiative transfer framework – Pandora – that em-
ploys RADMC-3D (Dullemond 2012) for a self-consistent determina-
tion of the dust temperature. With this pipeline, we have set-up a
single model that consistently reproduces the thermal dust and free-
free continuum emission of Sgr B2 spanning four orders of magnitude
in spatial scales (0.02–45 pc) and two orders of magnitude in frequency
(20–4000GHz).
1 Introduction
The giant molecular cloud complex Sgr B2 is located at a distance of 8.5 pc (Reid
et al. 2014), close to the Galactic center. The entire complex contains a total
gas mass of 107M, distributed in a large envelope of ∼22 pc in radius (Lis &
Goldsmith 1989). Despite the large mass reservoir, star formation seems to be
mainly occuring in the two hot molecular cores Sgr B2(N) and Sgr B2(M). These
two sites of active star formation are located at the center of the envelope occupying
1 I. Physikalisches Institut, Universita¨t zu Ko¨ln, Zu¨lpicher Straße 77, 50937 Ko¨ln, Germany
2Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor,
48109-1042, USA
3Max-Planck-Institut fu¨r Radioastronomie, Auf dem Hu¨gel 69, 53121 Bonn, Germany
4 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universite´s,
UPMC Univ. Paris 06, 75014 Paris, France
5 California Institute of Technology, Pasadena, CA 91125, USA
6 INAF – Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133
Roma, Italy
7Department of Astronomy, Yunnan University, and Key Laboratory of Astroparticle Physics
of Yunnan Province, Kunming 650091, China
c© EAS, EDP Sciences 2016
DOI: 10.1051/eas/1575030
172 Conditions and Impact of Star Formation
Fig. 1. Flowchart of the radiative transfer modeling framework Pandora.
an area of around 2 pc in radius. They contain more than 50 high-mass stars with
spectral types ranging from O5 to B0, and constitute one of the best laboratories
for the search of new chemical species in the Universe.
The modeling of the thermal dust and free-free continuum emission of Sgr B2
provides us with the three-dimensional model of the structure (i.e. density distri-
bution) of this region. For this, multi-wavelength and multi-scale data is crucial
to properly constrain the structure. In the next step this will enable us to model
the line shapes (kinematics) and thus constrain molecular gas properties, such as
the gas velocity ﬁeld, molecular abundances, etc..
2 Pandora
Pandora follows the ﬂowchart in Figure 1 for the continuum modeling. As part
of the framework, we employ the 3-dimensional, publicly available radiative trans-
fer program RADMC-3D (Dullemond 2012), Miriad for the post-processing, and
MAGIX (Mo¨ller et al. 2013) for the optimization of the model input parameters.
For a full description of Pandora see (Schmiedeke et al. 2016).
In short, we use the adaptive mesh reﬁnement (AMR) technique to locally
increase the spatial resolution on a cell-by-cell basis. We include stars as heating
sources. For each known Hii region, we place a corresponding star in the model.
We then sprinkle low-mass stars according to Kroupa’s initial mass function (IMF).
The overall density structure is obtained by a superposition of the Plummer-like
density proﬁles of all dust cores. We consider Hii regions as Stro¨mgren spheres,
i.e. as fully ionized, spherical regions of uniform electron density with no dust.
The dust temperature is calculated self-consistently by RADMC-3D. We use one
dust species (dust without grain mantles and no coagulation from Ossenkopf &
Henning 1994) and do not include scattering events.
In order to compare synthetic maps with real observations, telescope speciﬁc
post-processing is necessary, e.g. performing uv-ﬁltering in case of interferometric
observations. Since three-dimensional modeling has intrinsically many free para-
meters, we focused on deriving the density ﬁeld for a ﬁxed dust setup. Exploring
the complete parameter space of all free parameters is prohibitive in terms of com-
puting time. Hence, we used a hybrid approach. First, we derive a good guess by
varying parameters by hand and judging the quality visually (χ2-by-eye). We then
employ MAGIX (Mo¨ller et al. 2013) to iteratively search for the best solution.
A. Schmiedeke et al.: Continuum Radiative Transfer Modeling of Sgr B2 173
Fig. 2. Sgr B2, large scale continuum maps (see Schmiedeke et al. 2016).
3 Results
We show the best-ﬁt model for the large-scale envelope and the small-scale struc-
ture around Sgr B2(M) in Figures 2 and 3, respectively. In these ﬁgures, the top
row shows the observational data while the bottom row shows the synthetic maps.
For the extended envelope, we ﬁnd a superposition of dust density structures that
are elongated along the line-of-sight necessary to match the observations. To
reconstruct the spectral energy distribution, we locate Sgr B2(N) along the line-
of-sight behind the plane containing Sgr B2(M). In both, the observed and the
synthetic maps, the intensity of Sgr B2(M) is stronger than Sgr B2(N) except at
870µm. Furthermore the observed maps show a clear asymmetry, i.e. the contin-
uum emission shows a faint extension to the west and a sharp edge to the east.
So while incapable of reproducing the absolute intensities, this model reproduces
the general behaviour. A detailed study of the envelope structure will likely lead
to a more clumpy distribution of the dust density and provide an improved ﬁt.
In addition, we show in Figure 3 the physical structure as three diﬀerent
density-isocontours, 10−17.5, 10−18, and 10−19 g cm−3, coloured with the dust tem-
perature. The surface of the many Hii regions in Sgr B2(M) gets heated by the
embedded protostellar objects up to temperatures of more than 600K. Assuming
a gas-to-dust ratio of 100 (Hildebrand 1983), we convert dust mass into gas mass
and together with the stellar mass distribution we derive an average star formation
eﬃciency of 50% for Sgr B2(M) and 5% for Sgr B2(N), indicating that Sgr B2(M)
has already dispersed a lot of gas.
174 Conditions and Impact of Star Formation
Fig. 3. Left and middle: interferometric continuum maps from the VLA (left) and the
SMA (middle) of Sgr B2(M). The observed maps are located in the top row, the synthetic
maps in the bottom row. Observed and synthetic map share the same colorscale. Right :
impression of the 3d view of the dust density distribution in Sgr B2(M). The model is
cut open at the model center to allow a view inside (see Schmiedeke et al. 2016).
4 Conclusions
We successfully reconstruct a possible three dimensional density distribution of
Sgr B2, recovering the continuum structures covering a wide frequency range
(20–4000GHz) on scales from 100 au to 45 pc. We derive a total dust mass of
105M for Sgr B2 and ﬁnd stellar masses of 20700M and 2400M, as well as
luminosities of 1.2 × 107 L and 1.8 × 106 L for Sgr B2(M) and Sgr B2(N), re-
spectively. The average star formation eﬃciency is low (5%) for Sgr B2(N) and
comparatively high (50%) for Sgr B2(M), indicating that Sgr B2(M) has already
dispersed a lot of gas.
References
Dullemond, C.P., 2012, RADMC-3D, ASCL
Hildebrand, R.H., 1983, QJRAS, 24, 267
Mo¨ller, T., et al., 2013, A&A, 549, A21
Ossenkopf, V., & Henning, T., 1994, A&A, 291, 943
Reid, M.J., et al., 2014, ApJ, 783, 130
Schmiedeke, A., et al., 2016, A&A, in press


Conditions and Impact of Star Formation
R. Simon, R. Schaaf and J. Stutzki (eds)
EAS Publications Series, 75-76 (2015) 171–174
CONTINUUM RADIATIVE TRANSFER MODELING OF
SAGITTARIUS B2
A. Schmiedeke1, P. Schilke1, Th. Möller1, Á. Sánchez-Monge1,
E. Bergin2, C. Comito1, T. Csengeri3, D.C. Lis4,5, S. Molinari6,
S.L. Qin7 and R. Rolffs1
Abstract. We present results from radiative transfer modeling of the
continuum emission towards Sagittarius B2 (hereafter Sgr B2). We
have developed a radiative transfer framework – Pandora – that em-
ploys RADMC-3D (Dullemond 2012) for a self-consistent determina-
tion of the dust temperature. With this pipeline, we have set-up a
single model that consistently reproduces the thermal dust and free-
free continuum emission of Sgr B2 spanning four orders of magnitude
in spatial scales (0.02–45 pc) and two orders of magnitude in frequency
(20–4000GHz).
1 Introduction
The giant molecular cloud complex Sgr B2 is located at a distance of 8.5 pc (Reid
et al. 2014), close to the Galactic center. The entire complex contains a total
gas mass of 107M, distributed in a large envelope of ∼22 pc in radius (Lis &
Goldsmith 1989). Despite the large mass reservoir, star formation seems to be
mainly occuring in the two hot molecular cores Sgr B2(N) and Sgr B2(M). These
two sites of active star formation are located at the center of the envelope occupying
1 I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
2Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor,
48109-1042, USA
3Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
4 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités,
UPMC Univ. Paris 06, 75014 Paris, France
5 California Institute of Technology, Pasadena, CA 91125, USA
6 INAF – Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133
Roma, Italy
7Department of Astronomy, Yunnan University, and Key Laboratory of Astroparticle Physics
of Yunnan Province, Kunming 650091, China
c© EAS, EDP Sciences 2016
DOI: 10.1051/eas/1575030
172 Conditions and Impact of Star Formation
Fig. 1. Flowchart of the radiative transfer modeling framework Pandora.
an area of around 2 pc in radius. They contain more than 50 high-mass stars with
spectral types ranging from O5 to B0, and constitute one of the best laboratories
for the search of new chemical species in the Universe.
The modeling of the thermal dust and free-free continuum emission of Sgr B2
provides us with the three-dimensional model of the structure (i.e. density distri-
bution) of this region. For this, multi-wavelength and multi-scale data is crucial
to properly constrain the structure. In the next step this will enable us to model
the line shapes (kinematics) and thus constrain molecular gas properties, such as
the gas velocity field, molecular abundances, etc..
2 Pandora
Pandora follows the flowchart in Figure 1 for the continuum modeling. As part
of the framework, we employ the 3-dimensional, publicly available radiative trans-
fer program RADMC-3D (Dullemond 2012), Miriad for the post-processing, and
MAGIX (Möller et al. 2013) for the optimization of the model input parameters.
For a full description of Pandora see (Schmiedeke et al. 2016).
In short, we use the adaptive mesh refinement (AMR) technique to locally
increase the spatial resolution on a cell-by-cell basis. We include stars as heating
sources. For each known Hii region, we place a corresponding star in the model.
We then sprinkle low-mass stars according to Kroupa’s initial mass function (IMF).
The overall density structure is obtained by a superposition of the Plummer-like
density profiles of all dust cores. We consider Hii regions as Strömgren spheres,
i.e. as fully ionized, spherical regions of uniform electron density with no dust.
The dust temperature is calculated self-consistently by RADMC-3D. We use one
dust species (dust without grain mantles and no coagulation from Ossenkopf &
Henning 1994) and do not include scattering events.
In order to compare synthetic maps with real observations, telescope specific
post-processing is necessary, e.g. performing uv-filtering in case of interferometric
observations. Since three-dimensional modeling has intrinsically many free para-
meters, we focused on deriving the density field for a fixed dust setup. Exploring
the complete parameter space of all free parameters is prohibitive in terms of com-
puting time. Hence, we used a hybrid approach. First, we derive a good guess by
varying parameters by hand and judging the quality visually (χ2-by-eye). We then
employ MAGIX (Möller et al. 2013) to iteratively search for the best solution.
A. Schmiedeke et al.: Continuum Radiative Transfer Modeling of Sgr B2 173
Fig. 2. Sgr B2, large scale continuum maps (see Schmiedeke et al. 2016).
3 Results
We show the best-fit model for the large-scale envelope and the small-scale struc-
ture around Sgr B2(M) in Figures 2 and 3, respectively. In these figures, the top
row shows the observational data while the bottom row shows the synthetic maps.
For the extended envelope, we find a superposition of dust density structures that
are elongated along the line-of-sight necessary to match the observations. To
reconstruct the spectral energy distribution, we locate Sgr B2(N) along the line-
of-sight behind the plane containing Sgr B2(M). In both, the observed and the
synthetic maps, the intensity of Sgr B2(M) is stronger than Sgr B2(N) except at
870µm. Furthermore the observed maps show a clear asymmetry, i.e. the contin-
uum emission shows a faint extension to the west and a sharp edge to the east.
So while incapable of reproducing the absolute intensities, this model reproduces
the general behaviour. A detailed study of the envelope structure will likely lead
to a more clumpy distribution of the dust density and provide an improved fit.
In addition, we show in Figure 3 the physical structure as three different
density-isocontours, 10−17.5, 10−18, and 10−19 g cm−3, coloured with the dust tem-
perature. The surface of the many Hii regions in Sgr B2(M) gets heated by the
embedded protostellar objects up to temperatures of more than 600K. Assuming
a gas-to-dust ratio of 100 (Hildebrand 1983), we convert dust mass into gas mass
and together with the stellar mass distribution we derive an average star formation
efficiency of 50% for Sgr B2(M) and 5% for Sgr B2(N), indicating that Sgr B2(M)
has already dispersed a lot of gas.
174 Conditions and Impact of Star Formation
Fig. 3. Left and middle: interferometric continuum maps from the VLA (left) and the
SMA (middle) of Sgr B2(M). The observed maps are located in the top row, the synthetic
maps in the bottom row. Observed and synthetic map share the same colorscale. Right :
impression of the 3d view of the dust density distribution in Sgr B2(M). The model is
cut open at the model center to allow a view inside (see Schmiedeke et al. 2016).
4 Conclusions
We successfully reconstruct a possible three dimensional density distribution of
Sgr B2, recovering the continuum structures covering a wide frequency range
(20–4000GHz) on scales from 100 au to 45 pc. We derive a total dust mass of
105M for Sgr B2 and find stellar masses of 20700M and 2400M, as well as
luminosities of 1.2 × 107 L and 1.8 × 106 L for Sgr B2(M) and Sgr B2(N), re-
spectively. The average star formation efficiency is low (5%) for Sgr B2(N) and
comparatively high (50%) for Sgr B2(M), indicating that Sgr B2(M) has already
dispersed a lot of gas.
References
Dullemond, C.P., 2012, RADMC-3D, ASCL
Hildebrand, R.H., 1983, QJRAS, 24, 267
Möller, T., et al., 2013, A&A, 549, A21
Ossenkopf, V., & Henning, T., 1994, A&A, 291, 943
Reid, M.J., et al., 2014, ApJ, 783, 130
Schmiedeke, A., et al., 2016, A&A, in press


English

We present results from radiative transfer modeling of the continuum emission towards Sagittarius B2 (hereafter Sgr B2). We have developed a radiative transfer framework – Pandora – that employs RADMC-3D (Dullemond 2012) for a self-consistent determination of the dust temperature. With this pipeline, we have set-up a single model that consistently reproduces the thermal dust and free-free continuum emission of Sgr B2 spanning four orders of magnitude in spatial scales (0.02–45 pc) and two orders of magnitude in frequency (20–4000 GHz)

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Continuum radiative transfer Modeling of Sagittarius B2

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