A 3mm line survey of Barnard 1b:
looking into the chemical content of a protostellar system
N.
(1)INAF-IRA
(1)
Marcelino ,
J.
(2)
Cernicharo ,
M.
(3)
Gerin ,
A.
(4)
Fuente ,
E.
(5)
Roueff
(Italy); (2)ICMM-CSIC (Spain); (3)LERMA, Obs. Paris (France); (4)OAN-IGN (Spain); (5)LUTH, Obs. Paris (France)
Introduction
Spectral line surveys are the best tool to obtain a complete view of the molecular
complexity of dark clouds, the sites of low-mass star formation and future planetary
systems like ours. The lack of internal heating sources and violent physical processes,
like shocks, make these dense and quiescent cores the best sites to explore and to
model interstellar gas-phase chemistry and molecular depletion into the dust grain
surfaces. However cold dark clouds have demonstrated to be less chemically
simple than previously thought. In a previous spectral scan between 86—92 GHz
towards four dark clouds, B1b was found to be a remarkable core with a rich
variety of molecular species. In particular, this source stands out for its wealth in
deuterated and complex organic molecules, some of which detected for the first
time in B1b (D2CS, Marcelino et al. 2005; 15NH2D, Gerin et al. 2009; HCNO, Marcelino
et al. 2009; HOCN, Marcelino et al. 2010). With the upgrade in 2011 of the IRAM 30m
equipment, we decided to extend this study to the whole 3mm band.
B1b is part of the Barnard 1 cloud in the Perseus complex, and contains three
sources: B1b-W, an infrared source detected by Spitzer (Jørgensen et al. 2006); and
two compact (sub)millimeter continuum sources, B1b-N and B1b-S (Huang & Hirano
2013). All these sources are deeply embedded in a surrounding protostellar core,
that seems essentially unaffected by the compact sources as shown by its large
column density, N(H2) ~ 1023 cm-2 and cold kinetic temperature, Tkin = 12 K (Lis et al.
2010). However recent observations indicate B1b could be in a very early stage of
the star formation process, with B1b-N and B1b-S proposed to be first hydrostatic
cores (FHSC, Pezzuto et al. 2012, Huang & Hirano 2013, Hirano & Liu 2014, Gerin et
al. 2015, and poster 73). It is possible the peculiar complex molecular content of
B1b could be related with its evolution towards the formation of protostars.
Figure 1. Full spectral scan between 82.5—117.5 GHz observed at IRAM 30m.
Observations were performed in frequency switching mode, using the EMIR
3mm receivers and the fast Fourier Transform Spectrometers in narrow mode
(50 kHz of spectral resolution).
Results
Figure 1 shows the full observed spectra towards B1b, where the most prominent features have been labelled. We have reached a sensitivity of 4-5 mK in a 50 kHz
spectral resolution channel (~ 0.17—0.12 km s-1), allowing the detection of many features below 50 mK (see bottom panel in Fig. 1). Among them, new molecules and
complex species were found, such as CH3O, CH3OCH3, CH3OCHO, CH2OCH2, HCOOH, etc. (see Cernicharo et al. 2012). The first results of this survey presented by
Cernicharo et al. (2012) have already shown the importance of reaching a good detection limit for these cold clouds. So far we have identified 451 lines from 111
molecular species including isotopologues, while more than 100 U-lines remain. Table 1 shows the list of the observed molecules. As already mentioned B1b shows a
particularly high deuterium fractionation, and we have detected several singly and doubly deuterated species. The largest molecule in our survey is dimethyl ether with
nine atoms, which is detected for the first time in this source and, together with other species such as methyl formate, considered to be a hot-core-like molecule. Also
noteworthy is the presence of many sulfurated species, including SO and SO2 typically associated with outflowing material.
Table 1. List of the observed molecules, not including rare isotopologues.
2 atoms
CN
CO
CS
SiO
PN
NS
SO
SO+
3 atoms
CCH
HCN
HNC
HCO
HCO+
NNH+
SD2
CCO
HCS+
DCS+
CCS
OCS
SO2
4 atoms
NH2D
NHD2
D2CO
l-C3H
c-C3H
HNCO
HCNO
HOCN
DNCO
HOCO+
DOCO+
H2CS
HDCS
D2CS
CCCN
CCCO
HSCN
CCCS
5 atoms
CH3O
l-C3H2
l-C3HD
c-C3H2
c-C3HD
c-C3D2
H2CCO
HDCCO
HCOOH
C4H
HC3N
HCCNC
CCCNH
DC3N
6 atoms
CH3OH
CH2DOH
CHD2OH
CH3OD
CH3CN
CH3SH
l-C4H2
HC3NH+
HCCCHO
7 atoms
CH3CCH
CH2DCCH
CH3CHO
CH2OCH2
CH2CHCN
8 atoms
CH3OCHO
9 atoms
CH3OCH3
Figure 2. Computed abundances for the observed molecules.
References
Column densities and abundances
Gaussian profiles were fitted to all the lines to obtain intensities, line widths and velocities.
In many cases multiple gaussians were used, mostly due to the existence of two velocity
components in B1b (at 6.5 and 7.0 km s-1). A more detailed analysis on each component
will be performed, but in the following we consider the total line profile. Using the
obtained integrated intensities and the rotational diagrams technique, we computed
column densities for all the observed species. From these diagrams, which represent the
column density of each rotational state versus its energy, we can derive the rotational
temperature and the total column density of the molecule. This technique assumes local
thermodynamic equilibrium, optically thin lines, and that the source fills the beam. For
those species expected to be optically thick, we used the rarer isotopologues and the
following isotopic ratios: 12C/13C = 60, 14N/15N = 300, 16O/18O = 600, 32S/34S = 22.5. For most
species we were able to derive both the rotational temperature and column density, but
in some cases (only one line observed, weak detections, etc.) we assumed Trot = 10 K.
This assumption is consistent with the obtained values for other molecules (~ 6-14 K) and
was also used in previous works (e.g. Cernicharo et al. 2012).
Molecular abundances with respect to H2 were computed using N(H2) = 7.6 x 1022 cm-2
(Daniel et al. 2013). Figure 2 shows the resulting abundances, which are mostly between
10-9—10-11. In the figure the Y axis range has been chosen to plot more clearly all the
species, thus not showing the full CO abundance ([CO]/[H2] ~ 5 x 10-5). After CO, other
species showing high abundances are CN, CCH, HNC, HCO+, and CH3OH (~10-8—10-9).
It is also worth noting the large abundances of deuterated species like NH2D, NHD2, and
D2CO; and sulfur bearing molecules such as NS, SO, SO+, CCS, OCS and H2CS. As
mentioned above and discussed in Cernicharo et al.(2012), the most surprising results of
this sensitive survey is the detection
of complex organic molecules
typically associated with hot cores.
These species show abundances of
10-10—10-11, with CH3OCHO and
CH3OCH3 showing the largest ones
(~ 2-4 10-2 wrt CH3OH).
A more detailed analysis of each
species is on going, which relative
abundance ratios could provide
valuable clues to link the physical
and chemical evolutionary stage
of B1b, a core in between the
prestellar phase and the formation
of Class 0 protostars.
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