Methanol emission from low mass protostars

Abstract

We present observations of methanol lines in a sample of Class 0 low mass protostars. Using a 1-D radiative transfer model, we derive the abundances in the envelopes. In two sources of the sample, the observations can only be reproduced by the model if the methanol abundance is enhanced by about two order of magnitude in the inner hot region of the envelope. Two other sources show similar jumps, although at a lower confidence level. The observations for the other three sources are well reproduced with a constant abundance, but the presence of a jump cannot be ruled out. The observed methanol abundances in the warm gas around low mass protostars are orders of magnitude higher than gas phase chemistry models predict. Hence, in agreement with other evidences, this suggest that the high methanol abundance reflects recent evaporation of ices due to the heating by the newly formed star. The observed abundance ratios of CH3 OH, H2 CO, and CO are in good agreement with grain surface chemistry models. However, the absolute abundances are more difficult to reproduce and may point towards the presence of multiple ice components in these regions.

Full text

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arXiv:astro-ph/0507172v2 15 Jul 2005
Astronomy & Astrophysics manuscript no. aa˙ch3oh
(DOI: will be inserted by hand later)
February 5, 2008
CH3OH abundance in low mass protostars
S. Maret1⋆, C. Ceccarelli1, A. G. G. M. Tielens2, E. Caux3, B. Lefloch1, A. Faure1, A. Castets4, and D. R. Flower5
1Laboratoire d’Astrophysique, Observatoire de Grenoble, BP 53, F-38041 Grenoble Cedex 09, France
2Space Research Organization of the Netherlands, PO Box 800, 9700 AV Groningen, The Netherlands
3Centre d’Etude Spatiale des Rayonnements, CESR/CNRS-UPS, BP 4346, F-31028 Toulouse Cedex 04, France
4Observatoire de Bordeaux, BP 79, F-33270 Floirac, France
5Physics Department, The University, Durham DH1 3LE, UK
Received / Accepted
Abstract. We present observations of methanol lines in a sample of Class 0 low mass protostars. Using a 1-D radiative transfer
model, we derive the abundances in the envelopes. In two sources of the sample, the observations can only be reproduced by
the model if the methanol abundance is enhanced by about two order of magnitude in the inner hot region of the envelope. Two
other sources show similar jumps, although at a lower confidence level. The observations for the other three sources are well
reproduced with a constant abundance, but the presence of a jump cannot be ruled out. The observed methanol abundances in
the warm gas around low mass protostars are orders of magnitude higher than gas phase chemistry models predict. Hence, in
agreement with other evidence, this suggests that the high methanol abundance reflects recent evaporation of ices due to the
heating by the newly formed star. The observed abundance ratios of CH3OH, H2CO and CO are in good agreement with grain
surface chemistry models. However, the absolute abundances are more difficult to reproduce and may indicate the presence of
multiple ice components in these regions.
Key words. ISM: abundances - ISM: molecules - Stars: formation
1. Introduction
During the formation of a star, the gas undergoes impor-
tant physical and chemical changes. In the prestellar phase,
the gas is heavily depleted by accretion on grain mantles.
When the gravitational collapse starts, the protostar warms
the gas while ice mantle molecules are released into the gas
phase. These released molecules trigger the formation of more
complex molecules through rapid reactions in the warm gas
(Charnley et al. 1992).
While the importance of ice mantle evaporation and hot
core chemistry around high mass protostars was known well
over a decade ago, the existence of such regions in low mass
protostars has been established more recently. Ceccarelli et al.
(2000a,b) have shown that H2O and H2CO abundances in the
lowmass protostarIRAS16293-2422areincreasedinthewarm
and dense inner part of the circumstellar envelope. In this re-
gion, H2O and H2CO are evaporated from the grain mantles
and are injected in the gaseous phase. These findings were
later confirmed by Sch¨ oier et al. (2002), who found strong ev-
idence for the increase of several other molecular abundances
(e.g. CH3OH, SO and SO2). Evaporation of H2O has also been
Send
smaret@umich.edu
⋆Present address: University of Michigan, Department of
Astronomy, 500 Church St., Ann Arbor MI 48109-1042, USA
offprintrequests to: S´ ebastienMaret, e-mail:
shown around one other Class 0 protostar, NGC1333-IRAS4
(Maret et al. 2002). Recently, Maret et al. (2004, Paper I) car-
ried out a survey of the emission of H2CO in a sample of nine
low mass protostars,andconcludedthat inall theobservedpro-
tostars but one, H2CO is between two and three orders of mag-
nitude more abundant in the inner part of the envelope than in
the outer part, suggesting that the evaporation of grain man-
tles is a common phenomenon in the inner parts of low mass
protostars.
While the importance of grain mantle evaporation is now
wellestablishedinlowmassprotostars,thequestionofwhether
these molecules will form “daughter” molecules by hot core
chemistry (Charnley et al. 1992) is still open. From a theoreti-
cal point of view, if the protostar is undergoing collapse, only
very rapidly formed second-generationcomplexmolecules can
be produced. As noted by Sch¨ oier et al. (2002) if the protostar
is in free fall, the evaporated molecules will fall on the cen-
tral object in a few hundreds years. Hence, while evaporation
can still occur on such a rapid timescale, the composition of
the gas phase is then ‘frozen’ in, because the dynamical time
scale is much shorter than the time needed to form second
generation molecules (Rodgers & Charnley 2003). However,
observationally, the recent detections of complex O and N
bearing molecules – typical of massive hot cores – towards
IRAS162293-2422(Cazaux et al. 2003; Bottinelli et al. 2004b)
and NGC1333-IRAS4A (Bottinelli et al. 2004a) are challeng-

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2S. Maret et al.: CH3OH in low mass protostars
ing these theoretical concepts, emphasising that the chemistry
in low mass protostar envelopes is still poorly understood and
needs to be further investigated.
Methanol is particularly suited for the study of hot cores
around low mass protostars, because it is relatively abundant
in grain mantles and dominates the organic content of evap-
oration zones. Indeed, such enrichments of methanol have
already been observed in the inner regions of high mass
protostars (van der Tak et al. 2000a), and in the gas shocked
by prostellar outflows (Bachiller et al. 1998; Buckle & Fuller
2002). Moreover, methanol, being a slightly asymmetric-top
molecule, can be used to probe a large range of physical con-
ditions, both in density and temperature (e.g. Leurini et al.
2004). The recent computations of collisional rate coefficients
of methanol with para-H2(Pottage et al. 2004) make precise
determination of these conditions possible.
In this paper, observations of methanol transitions from
low mass protostars are presented. A detailed radiative trans-
fer model is used to derive the methanol abundance inside the
protostellar envelopes. Finally, the methanol abundances in the
envelopesare comparedto those in otherenvironments,andthe
implications for the formation of this molecule are discussed.
2. Observations
Six Class 0 protostars (see Table 1) were observed using the
Institut de Radio-Astronomie Millim´ etrique 30 meters tele-
scope (IRAM-30m)1, and the James Clerk Maxwell Telescope
(JCMT)2. The methanol 5K-4Ktransitions were observed with
the IRAM-30m telescope, while the 7K-6Kwere observed with
the JCMT. JCMT and CSO observations of IRAS16293-2422
from the literature were also used (van Dishoeck et al. 1995).
The IRAM-30m observations were obtained in November
2002. The A230 and B230 receivers were used simultaneously
with the autocorrelator, providing a spectral resolution of 0.5
km/s and a bandwidth of 320 MHz. The calibration and the
pointing were regularly checked and were found to be better
than 20% and 3′′respectively. For all the sources the obser-
vations were done in position switching mode, after checking
that the reference position was free of line emission. In order
to determine the spatial extent of the methanol emission, small
maps of 3 by 3 pixels were made, with a 10′′sampling.
The JCMT observations were obtained in September 2000
and from February 2001 to February 2003. The B3 receiver
was used with the digital autocorrelatorspectrometer (DAS) in
setup with a bandwith of 500 MHz and a spectral resolution
of 0.4 km/s. As for the IRAM-30m observations, the point-
ing and calibration were regularly checked and was found to
be better than 30% and 3′′respectively. A few lines already
observed by van Dishoeck et al. (1995) towards IRAS16293-
2422 were re-observed in September 2000. The lines fluxes
1IRAM is an international venture supported by INSU/CNRS
(France), MPG (Germany) and IGN (Spain).
2The JCMT is operated by the Joint Astronomy Center in
Hilo, Hawaii on behalf of the present organizations: The Particle
Physics and Astronomy Research Council in the United Kingdom,
the National Research Council of Canada and the Netherlands
Organization for Scientific Research.
from the two datasets were compared and were found to be
in agreement within 30%. The JCMT observations were done
in beam switching mode with a 180′′offset, large enough to
avoid line contamination by the outflows or by the cloud.
3. Results
Fig. 1 and 2 show the spectra towards the six observedsources.
Several 5-4 and 7-6 lines are detected in the bands observed
with IRAM and JCMT respectively.The lines with lowest level
energy (Eup∼ 40 - 70 K) in each band are the brightest in the
spectra and are detected toward all sources. On the contrary,
only a few lines with larger upper level energies are detected
in most sources, with NGC1333-IRAS2 showing the largest
number of detected methanol lines (26 lines, up to upper level
energies of 259 K). The relative intensity of the lines with
low level energy to lines with high level energy varies from
source to source, suggesting different excitation temperatures.
The richer methanol spectrum of NGC1333-IRAS2 is, in this
respect, likely due to a relatively larger excitation temperature
with respect to other sources.
The lines with lowest level energy show, in some sources,
bright high velocity wings, sometimes asymmetric. This
is the case for NGC1333-IRAS4A, NGC1333-IRAS4B and
NGC1333-IRAS2. In the other three sources (L1448-MM,
L1448-N and L1557-MM), all the lines, including those with
a low level energy, have relatively narrow Gaussian profiles
(between 1 and 4 kms−1) with much weaker broad wings, if
any. Usually, large asymmetric wings testify to the presence
of outflowing material. Indeed, the maps obtained at IRAM
in the direction of NGC1333-IRAS4A and NGC1333-IRAS2
confirm the presence of large scale outflows probed by the two
methanol lines with lowest level energy.
Fig. 3 shows the CH3OH 5K-4K emission map of
NGC1333-IRAS4A.Towardsthecentralposition,thetwo lines
with lowest level energy have Gaussian profiles with high ve-
locity wings. North and south of the source, the wings become
larger and asymmetric, red to the north-east and blue to the
south-west. Also, the emission of the two lines with lowest
level energyis not peakedin the central position,but rather10′′
south of it. The morphology of the observed emission is con-
sistent with the direction of the outflow elongating along the
north-south axis near NGC1333-IRAS4A3, as seen in CO 3-2,
CS 7-6 or SiO 2-1 emission (Blake et al. 1995; Lefloch et al.
1998).
The map of CH3OH 5K-4Kemission of NGC1333-IRAS2
is presented in Fig. 4. In the central position, the lines pro-
files aresimilar to thoseobservedinNGC1333-IRAS4A:a nar-
row Gaussian component, with high velocity wings, although
less intense. The emission becomes red-shifted north and east-
ward of the source, and decreases in the west and south di-
rection. Two outflows have been observed towards NGC1333-
IRAS2. A large scale outflow, oriented along the north-south
axis of the source, has been detected in CO 3-2 and 2-1 emis-
3There is an abrupt change of the position angle of the CO 3-2 flow
from approximatively 45◦on the large scale to 0◦near NGC1333-
IRAS4A (Blake et al. 1995).

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S. Maret et al.: CH3OH in low mass protostars3
Table 1. The observed sample.
Source
α(2000)
δ(2000)CloudDist.a
(pc)
220
220
220
220
220
325
160
Lbolb
(L⊙)
6
6
16
5
6
11
27
Menvb
(M⊙)
2.3
2.0
1.7
0.9
3.5
1.6
5.4
Lsmm/Lbolc
(%)
5
3
? 1
2
3
5
2
Tbolc
(K)
34
36
50
60
55
60
43
NGC1333-IRAS4A
NGC1333-IRAS4B
NGC1333-IRAS2
L1448-MM
L1448-N
L1157-MM
IRAS16293-2422
03:29:10.3
03:29:12.0
03:28:55.4
03:25:38.8
03:25:36.3
20:39:06.2
16:32:22.7
+31:13:31
+31:13:09
+31:14:35
+30:44:05
+30:45:15
+68:02:22
-24:38:32
Perseus
Perseus
Perseus
Perseus
Perseus
Isolated
ρ-Ophiuchus
aFrom Andr´ e et al. (2000), except for Perseus sources (ˇCernis 1990).
bFrom Jørgensen et al. (2002).
cFrom Andr´ e et al. (2000).
sion (Knee & Sandell 2000). A second outflow, oriented east
and west, is detected in SiO 2-1 (Blake 1996; Jørgensen et al.
2004a) and CS 3-2 (Langer et al. 1996). Strong CH3OH lines
have also been observed at the endpoint of this outflow (∼ 70′′
eastandwestofthecentralposition),wherethemethanolabun-
dance is enhanced by a factor of 300 (Bachiller et al. 1998). In
our map, low energy transitions trace the red lobe (towards the
north) of the outflow, as well as the blue (towards the west)
lobe of the east-west outflow, but only narrow and weak lines
are seen towards the east and south. On the contrary, lines with
high energy, also seen on the JCMT spectrum, appear only in
the central position.
Since this paperfocuses on the methanolemission fromthe
envelopes surrounding the protostars, in the following we will
restrain the discussion to this component only and will not an-
alyze further the outflow component, other than for disentan-
gling it from the envelope emission. In practice, we separated
the envelope emission, assumed to give rise to a Gaussian line
centered on the vlsrof the source, from the outflow emission,
assumed to be the residual of the Gaussian. Tables 4 to 10 list
the line fluxes and widths of the Gaussians, for each source, as
derivedfromthis analysis.As previouslysaid,inmostcases the
linesarerather“clean”Gaussianswithnarrowwidths,andlittle
contamination from wings. However, a few lines are strongly
contaminated by the outflow component,and such a separation
between the envelope and outflow components was not possi-
ble. In these cases, a single Gaussian was fitted on the entire
profile, therefore including both envelope and outflow contri-
butions. These lines are explicitly marked in Tables 4 to 10.
In thethreesourceswherethe outflowemissionis relatively
important (NGC1333-IRAS4A, IRAS4B and IRAS2), several
lines observedwith IRAM are significatively narrowerthan the
lines observed with the JCMT: between 1 and 4 kms−1the for-
mers, and up to 8 kms−1the latters (before outflow component
subtraction). Fig. 5 illustrates this difference, and presents two
methanol line profiles observed with IRAM-30m and JCMT
towards NGC1333-IRAS2. The line observed with the JCMT
clearly shows a red high velocity wing, while the IRAM spec-
trum does not. Likely, this difference is due to the larger beam
of JCMT (13′′) with respect to IRAM-30m (10′′). The former
probably picks up more extended emission from the outflows
than the latter. In addition, or alternatively, this difference can
be due to a slightly different pointing of the two telescopes.
The second panel of Fig. 5 shows the case of L1448-N,
where, on the contrary, the lines from the two telescopes are
very similar, confirming the lower contribution from the out-
flow to the line emission in this source. Fig. 5 also show a
formaldehyde line along with the two methanol IRAM and
JCMT lines. The figure shows that, in the case of NGC1333-
IRAS2, the JCMT methanol line is indeed much more contam-
inated by the outflow than the formaldehydeline, also obtained
at JCMT. However, inspection of the spectra obtained towards
L1448-N does not show any substantial difference, supporting
our conclusion that, in this source, the outflow does not play a
major role in the methanol emission.
In summary, profiles and maps of the detected methanol
lines suggest that in half of the sample sources (NGC1333-
IRAS4A, NGC1333-IRAS4B and NGC1333-IRAS2) the lines
with lowest level energy are strongly contaminated by the out-
flowing gas, whereas in the the lines with the highest level en-
ergy the problem is much less severe. In the remaining three
sources (L1448-MM, L1448-N and L1157-MM) no evidence
of severe outflow contamination has been observed in any
methanol line. For most of the lines showing contamination by
theoutflow,theoutflowemissionwasdisentangledfromtheen-
velopeemission.For a few lines markedin Tables 4 to 6, such a
separation was not possible. In all other cases, we are confident
that the quoted fluxes are representative of the emission from
the envelope only.
4. Modeling
4.1. Rotational diagrams
Rough estimates of the kinetic temperatures and column den-
sities have been obtained from the rotational diagrams of each
source (Fig. 6). The observations are reasonably well fitted by
a straight line, with some scattering, which is probably due to
opacity and non-LTE effects. No differences are observed be-
tween the JCMT and IRAM-30m observations, despite the dif-
ferent beam widths of the two instruments.
The derived rotational temperature and column densities
are summarizedin Table 2. The derivedcolumndensities range

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4 S. Maret et al.: CH3OH in low mass protostars
Fig.1. On source CH3OH 5K-4Kspectra.
between 0.5 and 8 × 1014cm−2. For most of the sources, rota-
tional temperatures are around 30 K. Two notable exceptions
are NGC1333-IRAS2 and IRAS16293-2422, whose rotational
temperatures are about 100 and 85 K respectively. That the
temperatures are larger in these two sources is not a surprise,
for they are the only two sources where several lines with high
level energy are detected, despite the fact that lines with low
level energy are not much brighter than in the other sources.
It is therefore very likely that the observed methanol lines in
NGC1333-IRAS2 and IRAS16293-2422 originate in the hot
region of the envelope.
Rotational temperatures should, however, be considered
with some caution, because they may not reflect the actual
kinetic temperatures. As noted by Bachiller et al. (1995) and
Buckle & Fuller (2002), methanol can be very sub-thermally
populated in interstellar conditions, and the rotational temper-
ature derived from methanol rotational diagram analysis could
be significantly lower than the kinetic temperature. Part of the
differences seen from one source to an other could be due to
differences in excitation. In the next section, a more detailed
analysis including non-LTE excitation is developed,in order to
take these effects into account.
4.2. Jump models
To determine more precisely the methanol abundance in the
envelopes, a detailed 1-D radiative transfer model has been de-
veloped.The model uses the escape probabilityformalism, and

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S. Maret et al.: CH3OH in low mass protostars5
Fig.2. As in Fig. 1 for CH3OH 7K-6K.
Table 2. Results from the rotational diagrams.
Source
Trot
(K)
24 ± 2
34 ± 4
101 ± 16
46 ± 10
19 ± 3
33 ± 25
84 ± 8
N(CH3OH)
(cm−2)
(5.1 ± 1.0) × 1014
(3.5 ± 0.8) × 1014
(3.4 ± 0.6) × 1014
(8.8 ± 2.4) × 1013
(9.4 ± 3.6) × 1013
(5.3 ± 4.9) × 1013
(8.1 ± 0.9) × 1014
NGC1333-IRAS4A
NGC1333-IRAS4B
NGC1333-IRAS2
L1448-MM
L1448-N
L1157-MM
IRAS16293-2422
has been presented in detail in Paper I. The energy levels and
Einstein coefficients were taken from the Cologne Molecular
Database Spectroscopy (M¨ uller et al. 2001). CH3OH colli-
sional rates with para H2from Pottage et al. (2004) were used.
The threefold hindering potential of the molecule leads to the
formation of A and E symmetry states (see Appendix A).
Because no radiative or collisional transitions are possible be-
tween the E and A symmetryspecies, these were modelledsep-
arately. The first 100 levels were considered for each symme-
try of the molecule. Finally, only the groundtorsional vibration
state (vt=0) was considered. The first excited state of the tor-
sional vibration (vt=1) is about 200 cm−1(∼ 290 K) above the
ground state. Levels with vt=1 might be excited and decay to
the torsional vibration ground state, but in a higher rotational
state. However, radiative excitation due to the dust is generally