HIFI Survey of Water Lines from Protostars, Astrofizyka i kosmologia
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219
HIFI SURVEY OF WATER LINES FROM PROTOSTARS
C.Ceccarelli
1
, A.Baudry
1
,E.Caux
2
, X.Tielens
3
,S.Bontemps
1
,J.Braine
1
, A.Castets
1
,M.Giard
2
,
F.Helmich
3
,F.Herpin
1
,T.Jacq
1
, C.Joblin
2
,S.Maret
2
, I.Ristorcelli
2
,andC.Vastel
2
1
Observatoire de Bordeaux, BP 89, 33270 Floirac, France
2
CESR CNRS-UPS, BP 4346, 31028 Toulouse cedex 04, France
3
SRON, PO Box 800, 9700 AV Groningen, the Netherlands
Abstract
Since water is very abundant and easily excited in the
circumstellar environments, water lines are an extremely
useful tool to probe the innermost regions of the envelopes
surrounding low mass protostars as well as the immediate
environment of compact and ultra-compact HII regions.
In this contribution we discuss the importance to carry
out a systematic survey of selected water lines in low and
high mass protostars with the high resolution spectrom-
eter HIFI on board FIRST. Given the large amount of
observing time necessary to survey a meaningful number
of lines in a meaningful statistical sample, this survey can
only be achieved by means of a KEY PROGRAM.
the warm inner regions of the envelopes surrounding low
luminosity protostars (Ceccarelli, Hollenbach & Tielens
1996; Doty & Neufeld 1997). These early predictions were
confirmed by the observations of strong water emission
in several low mass protostars carried out with the two
spectrometers on board the Infrared Space Observatory
(ISO) (e.g. Ceccarelli et al. 1999). The detailed analy-
sis of the water line spectrum of the solar type protostar
IRAS16293-2422allowedto reconstructthe physical struc-
ture ofits envelopeand to estimate the two key parameters
of the protostar: the mass of the central forming star and
its mass accretion rate (Ceccarelli et al. 2000). In addition,
the observed water lines enabled us to estimate the water
abundance in the cold outer envelope and in the warm
innermost regions enriched in water vapour, both because
of the evaporation of the water ice previously stored in
the cold grain mantles and because of eAcient chemical
reactions which lock most of the gaseous oxygen into wa-
ter molecules. What is important to note here is that the
model, corroborated by the H
2
O line observations, pre-
dicts the existence of a misty and warm region at about
150 AU where about 0.035 M
of water vapour collapses
towards the center. Once entered into the protoplanetary
disk, the water vapour may condense onto the cold grains,
which then aggregate to form icy planetesimals imprinted
with this first collapse phase memory (Chick & Cassen
1997).
Given the relatively low sensitivity, low spatial reso-
lution and low spectral resolution, the ISO observations
had some obvious limitations, which will be certainly over-
come by FIRST. With its higher sensitivity, and spatial
and spectral resolutions, FIRST will be able:
Key words: Stars: formation – Missions: FIRST
1. Introduction
Oxygen is the third most abundant element in the Uni-
verse,after HydrogenandHelium. Incoldmolecularclouds
it is mainly in the form of gaseous CO, O and H
2
O, (e.g.
van Dishoeck & Blake 1998; Caux et al. 1999) and in the
form of iced water coated around the grains (e.g. Tielens
1987). However, in star forming regions most of the oxy-
gen can be found in water, because of the evaporation of
the grain ice mantles and/or of the formation of water in
the gas phase via endothermic reactions (e.g. Kaufman &
Neufeld 1996; Ceccarelli, Hollenbach & Tielens 1996). Be-
cause of its high abundance and easy excitation in warm
interstellarand circumstellarenvironments water is a pow-
erful tool to probe astrophysical conditions in a broad va-
riety of sources, from protostars to molecular shocks. In
this contributionwe focus onthe casefor systematicobser-
vations of water lines in low and high mass protostars. We
specifically address the
capabilities of HIFI in ob-
taining high spectral and spatial resolutions observations
of the submm rotational transitions of water to study the
immediate environment of solar type protostars and com-
pact or ultra-compact HII regions.
a) to increase the sample of solar type protostars where
the study of water line emission is possible;
b) to disentangle the water emission associated with the
infalling gas against the emission due to shocked ma-
terial of the outflowing gas;
c) to resolve the line emission thus making possible kine-
matical studies: note that since high energy lying water
lines originate in the innermost regions, where the in-
fall velocity is the highest, they can be resolved by the
HIFI spectrometer and used to probe the infall;
d) and finally, to measure water abundances in the cold
envelopes with a sensitivity at least an order of mag-
nitude better than SWAS.
2. Low mass protostars
Water lines have been predicted to be the most abun-
dant oxygen bearing molecule and major gas coolants of
Proc. Symposium
‘The Promise of the Herschel Space Observatory’
12–15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
unique
220
C. Ceccarelli et al.
FIRST will be the only instrument able to observe eA-
ciently and routinelywater lines fromsolar type protostars
for many years to come.
In conclusion, the water lines observable with FIRST
are of paramount importance to derive key parameters for
theprocessofasolartypeprotostar:
–
the mass of the central forming stars and their accre-
tion rates;
–
the water abundance in the cold gas of the outer en-
velope and
–
the enrichmentof waterofthe innermostregions,which
may ultimately form planetary systems.
The knowledgeof these parametersin a statistically mean-
ingful and well selected sample of solar type protostars
would be invaluable to understand the processes that lead
to the formation of stars and planetary systems.
Table 1. Water line fluxes for a 30 L
protostar of 0.8 M
,
accreting at
3
×
10
−
5
M
yr
−
1
, and located at a distance of 160
pc (more details in Ceccarelli et al. 2000).
Frequency Transition o/p
Flux
(GHz)
erg s
−
1
cm
−
2
556.9 1
1
,
0
−
1
0
,
1
o
4.2E-13
752.0 2
1
,
1
−
2
0
,
2
p
1.5E-13
916.1 4
2
,
2
−
3
3
,
1
p
2.2E-14
970.3 5
2
,
4
−
4
3
,
1
p
1.7E-14
1097.4 3
1
,
2
−
3
0
,
3
o
1.3E-13
1153.1 3
1
,
2
−
2
2
,
1
o
2.1E-13
1158.3 6
3
,
4
−
5
4
,
1
o
1.2E-14
1162.9 3
2
,
1
−
3
1
,
2
o
1.1E-13
1228.8 2
2
,
0
−
2
1
,
1
p
1.0E-13
1296.4 8
2
,
7
−
7
3
,
4
o
3.6E-15
1440.9 7
2
,
6
−
6
3
,
3
p
5.5E-15
Line list
It is proposed to survey around a dozen water lines.
The lines are selected on the basis of theoretical predic-
tions (Ceccarelli, Hollenbach & Tielens 1996), supported
by the ISO observations in the few objects so far observed.
In order to make realistic predictions we ran a grid of
models for different protostar luminosities, masses, mass
accretion rates, water abundances etc. The selected lines
(Tab. 1) represent the result of this modeling effort. In
addition to H
1
2
O lines we propose to survey also two key
H
1
2
O lines, which will allow to derive in detail the col-
umn densities of the observed lines and therefore to bet-
ter reconstruct the envelope structure. Finally three HDO
lines will also be surveyed, appropriately selected to de-
rive the water deuteration through the envelope. Although
it is in principle possible to observe HDO from ground,
the very few observations available in literature show that
these observations are very diAcult. On the other hand
it is now clear that in solar type protostars molecules like
H
2
CO and NH
3
present extremely large degrees of deuter-
ation, more than 10% being in the deuterated forms of
these molecules (Ceccarelli et al. 1998; Loinard et al. 2000;
Roueff et al 2000). It will therefeore be extremely impor-
tant to study the deuterated form of H
2
Ointhesame
objects to understand the route of deuteration of these
molecules.
1602.2 4
1
,
3
−
4
0
,
4
p
8.0E-14
process of the low mass star formations, such as for is-
tance the sound speed in the parent cloud (e.g. Shu et al.
1987).
detec-
tion level over three channels (each of 1/3 of the predicted
line width). Using the HIFI available spectrometers we
propose to observe the lines In Table 1. The system tem-
peratures used to give the estimate of the observing time
are roughly those given as the “baseline” on page 48 of
the Scientific and Technical case for HIFI (Part I). Based
on the mentioned model predictions and such observing
time estimates to observe a meaningful set of water lines
in sources whose luminosity is about 1 L
will take around
four hours per source (no overheads are taken into account
yet). To survey the proposedstar forming regions will need
therefore about 300 hours of observations (with no over-
heads), which makes this study only possible via a KEY
PROGRAM.
σ
3. High mass protostars
Target list
It is proposed to survey some of the nearby low mass
star forming regions: Taurus,
Ophiuchus, Perseus, Ser-
pens, Orion and Chameleon are obvious examples. Over-
all these complexes contain about 80 embedded low lu-
minosity protostars, two thirds of them with luminosities
between1and3L
, and the remaining with higher lu-
minosities. Comparison of objects within the same region
have been already shown to be extremely useful to draw
evolutionary pictures (e.g. Saraceno et al. 1996; Bontemps
et al. 1996). Comparison between different regions high-
lights if and which macroscopic parameters enter in the
ρ
In the second part of this contribution we address the
question of the complex environment of embedded mas-
sive O- or B-type stars which have not yet fully dispersed
their natal cloud material. We specifically wish to use the
high spectral and spatial resolutions achieved with HIFI
in the submm rotational transitions of water to study the
immediate environment of compact or ultra-compact HII
regions.These regions are detected throughout the Galaxy
in both the radio and far infrared domains. Several of
them have been mapped with radio interferometers and
are known as luminous IRAS sources. Their total lumi-
Time estimate
To calculate the observing times we request a 2
HIFI Survey of Water Lines from Protostars
221
nosity is of the order of 10
3
L
or more and may reach
10
5
L
in the extreme case of W3(OH). They are often
associated with massive molecular clouds and masers and
they sometimes exhibit pronounced molecular outflows.
The spatial resolution of FIRST, of the order of 15
to
20
-or less for the highest frequencies of water-, does not
permit mapping the most compact HII regions although it
helps to discriminate the embedded object from the more
extended molecular environment. However, depending on
the target and the selected transition we expect to ob-
serve both absorption and emission in several transitions
of water, including maser emission, from the gas layers in
the neighbourhood of the HII region (see (i) and (ii) be-
low). In HII regions most of the oxygen is found in water
because icy grain mantles have been evaporated or be-
cause endothermic gas phase reactions dominate (see the
Introduction). Combined with easier excitation in warm
regions water is thus an eAcient coolant of the neutral
material surrounding the ionized gas associated with the
HII region.
(i)
Absorption
lines are most interesting because they
probe the compressed neutral regions lying along the
line of sight to the HII regions with excitation less
than the background emission. These lines thus nicely
compensate for the lack of spatial resolution.
(ii)
Emission
from warm water such as that detected to-
ward the Orion hot core (e.g. Cernicharo et al. 1999)
could be present in gas layers with temperatures of
order 100 K or above. Detectability of this gas de-
pends on the filling factor of the FIRST telescope beam
and does not specifically probe the gas immediately
against the HII region. On the other hand, the anoma-
lously excited lines leading to strong maser emission
because of shocks and collisional and infrared pumping
of water could also be detected despite beam dilution.
These masing transitions trace the warmest and dens-
est pockets of the gas or just trace the shocked regions;
they allow us to investigate the small-scale clumpiness
of the outer neutral gas layers against the HII region.
In conclusion, by surveying a number of submm transi-
tions of water in a large sample of sources throughout the
Galaxy we expect to derive a statistical view of the com-
plex physical conditions prevailing in the gas surrounding
embedded massive stars. We thus hope to bring new light
on the early evolutionary stages of embedded massive ob-
jects.
confirm this result and suggest that velocity and tempera-
ture gradients can be present in the cores (eg. Cesaroni et
al. 1998). Besides, ISO-SWS observations already showed
the potentiality of this technique (e.g. van Dishoeck &
Helmich 1996; Helmich et al. 1996; van Dishoeck et al.
1998; Wright et al. 2000). To identify the strongest H
2
O
absorption lines (Table 2) in the neutral gas in front of the
compact HII regions we used the model results by Doty
& Neufeld (1997) who explicitely computed the water line
spectra of massive protostars.
Table 2. Predicted water absorption lines for a cloud of mass
100 M
, illuminated by a central source of luminosity vary-
ing beteewn
10
3
and
10
5
L
, as computed by Doty & Neufeld
(1997).
Freq. Transition
16
O/
18
O
Line Luminosities (L
)
(GHz)
o/p
10
3
L
10
4
L
10
5
L
1101.7 1
1
,
1
−
0
0
,
0
18
O-p 4
.
1(
−
3) 9
3(
−
3) 2
.
5(
−
2)
1113.3 1
1
,
1
−
0
0
,
0
16
O-p 1
.
5(
−
2) 5
.
3(
−
2) 2
.
5(
−
2)
1655.9 2
1
,
2
−
1
0
,
1
18
O-o 1
.
5(
−
2) 5
5(
−
2) 1
.
4(
−
1)
1661.0 2
2
,
1
−
2
1
,
2
16
O-o 1
.
5(
−
2) 5
.
1(
−
2) 8
.
7(
−
2)
1669.9 2
1
,
2
−
1
0
,
1
16
O-o 3
.
8(
−
2) 1
.
9(
−
1) 7
.
4(
−
1)
1716.8 3
0
,
3
−
2
1
,
2
16
O-o 2
.
1(
−
2) 9
.
6(
−
2) 2
.
1(
−
1)
5
2
,
3
transition which is easily detected from the ground at 22
GHz in a broad variety of sources. The 22 GHz line al-
ways show maser emission and is nearly always associated
with regions of on-going star formation. In such regions
maser action is likely to be observed in several submm
lines. The strong 22 GHz maser emission can be explained
by the collisional pumping of dense neutral gas which has
been heated by shocks, either fast and dissociative (J-type
shocks; Elitzur et al. 1989) or slower and non-dissociative
(C-type shocks; Melnick et al. 1993) which can heat the
gas up to about 1000 K.
Following the results of Neufeld & Melnick (1991) we
expect rather strong maser emission from about a dozen of
lines in the frequency range 500 to 1500 GHz which is ac-
cessible to HIFI. The opacities of these lines depend on the
cloud geometry, the collision rates and the volumic den-
sity. In order to be independent both of the geometry and
the gas density we scale the submm transition opacities to
that of the 22 GHz transition. Using Neufeld & Melnick’s
model we select 7 submm transitions that are at least 0.1
to 0.2 times the 22 GHz line intensity and fall in the HIFI
receiver tuning range (Table 3). The opacities in the ta-
ble are relative to that at 22 GHz arbitrarily assumed
Line list
Absorption Lines
A first analysis of the gas lying in front of a limited num-
ber of compact HII regions has been made on the basis
of ground-based observations of several transitions of the
OH radical (eg. Wamsley et al. 1986; Baudry et al. 1993).
The data show that the kinetic temperature of the neutral
gas is of order 100 to 200 K. The ammonia observations
made in the hot cores associated with compact HII regions
.
.
Maser Lines
Many ortho and para rotational levels of water lie close
to each other and thus tend to be easily inverted. Ac-
cordingly, if these levels correspond to allowed radiative
transitions they can give rise to maser amplification. The
strongest water maser emission is that of the 6
1
,
6
−
222
to be -10 for a gas at 1000 K; other mm/submm transi-
tions observed from the ground are given in Table 3 for
comparison. Theoretical predictions of Neufeld & Melnick
(1991) show that maser emission is expected from several
transitions under a wide range of astrophysical conditions.
Therefore, FIRST observations of several submm transi-
tions of water combined with ground-state observations
(Table 3) can be used to constrain the physical conditions
in the H
2
O emitting layers. We wish to observe the seven
150 target sources in all tran-
sitions of water mentioned above. Using the goal system
temperatures expected in the various HIFI receiver bands
we believe that
∼
2 hours are required per source. For a
statistical analysis of a meaningful sample of sources taken
from our survey list we grossly estimate that more than
200-300 hours are required to cover all water lines of inter-
est. This can be achieved only if the FIRST Observatory
decides to support KEY PROGRAM observations.
Table 3. Predicted strong maser transitions in shocked regions.
4. CONCLUSIONS
Frequency Transition o/p Relative
(GHz)
Opacity
a
We discussed that water lines are the main gas coolants
in a variety of conditions typical of the gas surround-
ing protostars, both massive and solar type protostars.
For this reason water lines are unique tools to probe this
gas, its thermal, physical and chemical structure. In this
contribution we presented the case for a sistematic study
from the envelopes of low and high mass protostars. We
wish to emphasize once again that
FIRST will be the
onlyinstrumentabletoobservee0cientlyandrou-
tinely water lines from protostars for many years
to come.
Given the large observing time necessary to
gather a meaningful number of lines in a meaningful num-
ber of sources, the proposed program can only be achieved
if the FIRST Observatory decides to endorse
KEYPRO-
GRAMS
.
22.2
6
1
,
6
−
5
2
,
3
o
-10 (ground)
183.3
3
1
,
3
−
2
2
,
0
p
-4.6 (ground)
325.2
5
1
,
5
−
4
2
,
2
p
-4.9 (ground)
439.2
6
4
,
3
−
5
5
,
0
o
-3.6 (ground)
470.9
6
4
,
2
−
5
5
,
1
p
-1.1 (ground)
530.4 14
3
,
12
−
13
4
,
9
o
-1.7
620.7
5
3
,
2
−
4
4
,
1
o
-4.1
906.2
9
2
,
8
−
8
3
,
5
p
-1.2
970.3
5
2
,
4
−
4
3
,
1
p
-6.5
1158.3
6
3
,
4
−
5
4
,
1
o
-5.5
1440.8
7
2
,
6
−
6
3
,
3
p
-2.9
1542.0
6
3
,
3
−
5
4
,
2
p
-1.7
a
The 22 GHz opacity is arbitrary.
References
m. The 22 GHz
H
2
O maser sources are selected according to their total
integrated flux; we take here sources brighter than about
100 Jy km s
−
1
. The initial survey list includes about 150
objects. (It includes some major complexes such as W49,
W51 or W75; these regions contain several individual HII
regions which can sometimes be well separated by the
FIRST-HIFI beam.) Our source list will be further refined
using the most recent surveys of methanol masers in HII
regions (see also the contribution by Molinari et al. in this
volume).
Time estimate
At the moment it is diAcult to make an exact estimate
of the time required to observe with the HIFI available
µ
Baudry A. et al. 1993 A&A 271, 552
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spectrometers a total of
∼
lines in Table 3 not accessible from the ground. They trace
the densest gas layers and we expect that the ratio of some
of these lines together with ground-based observations will
give an estimate of the gas temperature.
Target list
We propose to survey all major HII regions also known
as bright IRAS sources. We concentrate our selection on
sources which have been detected in the 22 GHz line with
the Medicina 32-m survey (Palagi et al. 1993; Brand et al.
199; Codella et al. 1994). The IRAS Point Source Cat-
alogue is used to select (somewhat arbitrarily) objects
brighter than 1000 Jy at 60 and 100
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219
HIFI SURVEY OF WATER LINES FROM PROTOSTARS
C.Ceccarelli
1
, A.Baudry
1
,E.Caux
2
, X.Tielens
3
,S.Bontemps
1
,J.Braine
1
, A.Castets
1
,M.Giard
2
,
F.Helmich
3
,F.Herpin
1
,T.Jacq
1
, C.Joblin
2
,S.Maret
2
, I.Ristorcelli
2
,andC.Vastel
2
1
Observatoire de Bordeaux, BP 89, 33270 Floirac, France
2
CESR CNRS-UPS, BP 4346, 31028 Toulouse cedex 04, France
3
SRON, PO Box 800, 9700 AV Groningen, the Netherlands
Abstract
Since water is very abundant and easily excited in the
circumstellar environments, water lines are an extremely
useful tool to probe the innermost regions of the envelopes
surrounding low mass protostars as well as the immediate
environment of compact and ultra-compact HII regions.
In this contribution we discuss the importance to carry
out a systematic survey of selected water lines in low and
high mass protostars with the high resolution spectrom-
eter HIFI on board FIRST. Given the large amount of
observing time necessary to survey a meaningful number
of lines in a meaningful statistical sample, this survey can
only be achieved by means of a KEY PROGRAM.
the warm inner regions of the envelopes surrounding low
luminosity protostars (Ceccarelli, Hollenbach & Tielens
1996; Doty & Neufeld 1997). These early predictions were
confirmed by the observations of strong water emission
in several low mass protostars carried out with the two
spectrometers on board the Infrared Space Observatory
(ISO) (e.g. Ceccarelli et al. 1999). The detailed analy-
sis of the water line spectrum of the solar type protostar
IRAS16293-2422allowedto reconstructthe physical struc-
ture ofits envelopeand to estimate the two key parameters
of the protostar: the mass of the central forming star and
its mass accretion rate (Ceccarelli et al. 2000). In addition,
the observed water lines enabled us to estimate the water
abundance in the cold outer envelope and in the warm
innermost regions enriched in water vapour, both because
of the evaporation of the water ice previously stored in
the cold grain mantles and because of eAcient chemical
reactions which lock most of the gaseous oxygen into wa-
ter molecules. What is important to note here is that the
model, corroborated by the H
2
O line observations, pre-
dicts the existence of a misty and warm region at about
150 AU where about 0.035 M
of water vapour collapses
towards the center. Once entered into the protoplanetary
disk, the water vapour may condense onto the cold grains,
which then aggregate to form icy planetesimals imprinted
with this first collapse phase memory (Chick & Cassen
1997).
Given the relatively low sensitivity, low spatial reso-
lution and low spectral resolution, the ISO observations
had some obvious limitations, which will be certainly over-
come by FIRST. With its higher sensitivity, and spatial
and spectral resolutions, FIRST will be able:
Key words: Stars: formation – Missions: FIRST
1. Introduction
Oxygen is the third most abundant element in the Uni-
verse,after HydrogenandHelium. Incoldmolecularclouds
it is mainly in the form of gaseous CO, O and H
2
O, (e.g.
van Dishoeck & Blake 1998; Caux et al. 1999) and in the
form of iced water coated around the grains (e.g. Tielens
1987). However, in star forming regions most of the oxy-
gen can be found in water, because of the evaporation of
the grain ice mantles and/or of the formation of water in
the gas phase via endothermic reactions (e.g. Kaufman &
Neufeld 1996; Ceccarelli, Hollenbach & Tielens 1996). Be-
cause of its high abundance and easy excitation in warm
interstellarand circumstellarenvironments water is a pow-
erful tool to probe astrophysical conditions in a broad va-
riety of sources, from protostars to molecular shocks. In
this contributionwe focus onthe casefor systematicobser-
vations of water lines in low and high mass protostars. We
specifically address the
capabilities of HIFI in ob-
taining high spectral and spatial resolutions observations
of the submm rotational transitions of water to study the
immediate environment of solar type protostars and com-
pact or ultra-compact HII regions.
a) to increase the sample of solar type protostars where
the study of water line emission is possible;
b) to disentangle the water emission associated with the
infalling gas against the emission due to shocked ma-
terial of the outflowing gas;
c) to resolve the line emission thus making possible kine-
matical studies: note that since high energy lying water
lines originate in the innermost regions, where the in-
fall velocity is the highest, they can be resolved by the
HIFI spectrometer and used to probe the infall;
d) and finally, to measure water abundances in the cold
envelopes with a sensitivity at least an order of mag-
nitude better than SWAS.
2. Low mass protostars
Water lines have been predicted to be the most abun-
dant oxygen bearing molecule and major gas coolants of
Proc. Symposium
‘The Promise of the Herschel Space Observatory’
12–15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
unique
220
C. Ceccarelli et al.
FIRST will be the only instrument able to observe eA-
ciently and routinelywater lines fromsolar type protostars
for many years to come.
In conclusion, the water lines observable with FIRST
are of paramount importance to derive key parameters for
theprocessofasolartypeprotostar:
–
the mass of the central forming stars and their accre-
tion rates;
–
the water abundance in the cold gas of the outer en-
velope and
–
the enrichmentof waterofthe innermostregions,which
may ultimately form planetary systems.
The knowledgeof these parametersin a statistically mean-
ingful and well selected sample of solar type protostars
would be invaluable to understand the processes that lead
to the formation of stars and planetary systems.
Table 1. Water line fluxes for a 30 L
protostar of 0.8 M
,
accreting at
3
×
10
−
5
M
yr
−
1
, and located at a distance of 160
pc (more details in Ceccarelli et al. 2000).
Frequency Transition o/p
Flux
(GHz)
erg s
−
1
cm
−
2
556.9 1
1
,
0
−
1
0
,
1
o
4.2E-13
752.0 2
1
,
1
−
2
0
,
2
p
1.5E-13
916.1 4
2
,
2
−
3
3
,
1
p
2.2E-14
970.3 5
2
,
4
−
4
3
,
1
p
1.7E-14
1097.4 3
1
,
2
−
3
0
,
3
o
1.3E-13
1153.1 3
1
,
2
−
2
2
,
1
o
2.1E-13
1158.3 6
3
,
4
−
5
4
,
1
o
1.2E-14
1162.9 3
2
,
1
−
3
1
,
2
o
1.1E-13
1228.8 2
2
,
0
−
2
1
,
1
p
1.0E-13
1296.4 8
2
,
7
−
7
3
,
4
o
3.6E-15
1440.9 7
2
,
6
−
6
3
,
3
p
5.5E-15
Line list
It is proposed to survey around a dozen water lines.
The lines are selected on the basis of theoretical predic-
tions (Ceccarelli, Hollenbach & Tielens 1996), supported
by the ISO observations in the few objects so far observed.
In order to make realistic predictions we ran a grid of
models for different protostar luminosities, masses, mass
accretion rates, water abundances etc. The selected lines
(Tab. 1) represent the result of this modeling effort. In
addition to H
1
2
O lines we propose to survey also two key
H
1
2
O lines, which will allow to derive in detail the col-
umn densities of the observed lines and therefore to bet-
ter reconstruct the envelope structure. Finally three HDO
lines will also be surveyed, appropriately selected to de-
rive the water deuteration through the envelope. Although
it is in principle possible to observe HDO from ground,
the very few observations available in literature show that
these observations are very diAcult. On the other hand
it is now clear that in solar type protostars molecules like
H
2
CO and NH
3
present extremely large degrees of deuter-
ation, more than 10% being in the deuterated forms of
these molecules (Ceccarelli et al. 1998; Loinard et al. 2000;
Roueff et al 2000). It will therefeore be extremely impor-
tant to study the deuterated form of H
2
Ointhesame
objects to understand the route of deuteration of these
molecules.
1602.2 4
1
,
3
−
4
0
,
4
p
8.0E-14
process of the low mass star formations, such as for is-
tance the sound speed in the parent cloud (e.g. Shu et al.
1987).
detec-
tion level over three channels (each of 1/3 of the predicted
line width). Using the HIFI available spectrometers we
propose to observe the lines In Table 1. The system tem-
peratures used to give the estimate of the observing time
are roughly those given as the “baseline” on page 48 of
the Scientific and Technical case for HIFI (Part I). Based
on the mentioned model predictions and such observing
time estimates to observe a meaningful set of water lines
in sources whose luminosity is about 1 L
will take around
four hours per source (no overheads are taken into account
yet). To survey the proposedstar forming regions will need
therefore about 300 hours of observations (with no over-
heads), which makes this study only possible via a KEY
PROGRAM.
σ
3. High mass protostars
Target list
It is proposed to survey some of the nearby low mass
star forming regions: Taurus,
Ophiuchus, Perseus, Ser-
pens, Orion and Chameleon are obvious examples. Over-
all these complexes contain about 80 embedded low lu-
minosity protostars, two thirds of them with luminosities
between1and3L
, and the remaining with higher lu-
minosities. Comparison of objects within the same region
have been already shown to be extremely useful to draw
evolutionary pictures (e.g. Saraceno et al. 1996; Bontemps
et al. 1996). Comparison between different regions high-
lights if and which macroscopic parameters enter in the
ρ
In the second part of this contribution we address the
question of the complex environment of embedded mas-
sive O- or B-type stars which have not yet fully dispersed
their natal cloud material. We specifically wish to use the
high spectral and spatial resolutions achieved with HIFI
in the submm rotational transitions of water to study the
immediate environment of compact or ultra-compact HII
regions.These regions are detected throughout the Galaxy
in both the radio and far infrared domains. Several of
them have been mapped with radio interferometers and
are known as luminous IRAS sources. Their total lumi-
Time estimate
To calculate the observing times we request a 2
HIFI Survey of Water Lines from Protostars
221
nosity is of the order of 10
3
L
or more and may reach
10
5
L
in the extreme case of W3(OH). They are often
associated with massive molecular clouds and masers and
they sometimes exhibit pronounced molecular outflows.
The spatial resolution of FIRST, of the order of 15
to
20
-or less for the highest frequencies of water-, does not
permit mapping the most compact HII regions although it
helps to discriminate the embedded object from the more
extended molecular environment. However, depending on
the target and the selected transition we expect to ob-
serve both absorption and emission in several transitions
of water, including maser emission, from the gas layers in
the neighbourhood of the HII region (see (i) and (ii) be-
low). In HII regions most of the oxygen is found in water
because icy grain mantles have been evaporated or be-
cause endothermic gas phase reactions dominate (see the
Introduction). Combined with easier excitation in warm
regions water is thus an eAcient coolant of the neutral
material surrounding the ionized gas associated with the
HII region.
(i)
Absorption
lines are most interesting because they
probe the compressed neutral regions lying along the
line of sight to the HII regions with excitation less
than the background emission. These lines thus nicely
compensate for the lack of spatial resolution.
(ii)
Emission
from warm water such as that detected to-
ward the Orion hot core (e.g. Cernicharo et al. 1999)
could be present in gas layers with temperatures of
order 100 K or above. Detectability of this gas de-
pends on the filling factor of the FIRST telescope beam
and does not specifically probe the gas immediately
against the HII region. On the other hand, the anoma-
lously excited lines leading to strong maser emission
because of shocks and collisional and infrared pumping
of water could also be detected despite beam dilution.
These masing transitions trace the warmest and dens-
est pockets of the gas or just trace the shocked regions;
they allow us to investigate the small-scale clumpiness
of the outer neutral gas layers against the HII region.
In conclusion, by surveying a number of submm transi-
tions of water in a large sample of sources throughout the
Galaxy we expect to derive a statistical view of the com-
plex physical conditions prevailing in the gas surrounding
embedded massive stars. We thus hope to bring new light
on the early evolutionary stages of embedded massive ob-
jects.
confirm this result and suggest that velocity and tempera-
ture gradients can be present in the cores (eg. Cesaroni et
al. 1998). Besides, ISO-SWS observations already showed
the potentiality of this technique (e.g. van Dishoeck &
Helmich 1996; Helmich et al. 1996; van Dishoeck et al.
1998; Wright et al. 2000). To identify the strongest H
2
O
absorption lines (Table 2) in the neutral gas in front of the
compact HII regions we used the model results by Doty
& Neufeld (1997) who explicitely computed the water line
spectra of massive protostars.
Table 2. Predicted water absorption lines for a cloud of mass
100 M
, illuminated by a central source of luminosity vary-
ing beteewn
10
3
and
10
5
L
, as computed by Doty & Neufeld
(1997).
Freq. Transition
16
O/
18
O
Line Luminosities (L
)
(GHz)
o/p
10
3
L
10
4
L
10
5
L
1101.7 1
1
,
1
−
0
0
,
0
18
O-p 4
.
1(
−
3) 9
3(
−
3) 2
.
5(
−
2)
1113.3 1
1
,
1
−
0
0
,
0
16
O-p 1
.
5(
−
2) 5
.
3(
−
2) 2
.
5(
−
2)
1655.9 2
1
,
2
−
1
0
,
1
18
O-o 1
.
5(
−
2) 5
5(
−
2) 1
.
4(
−
1)
1661.0 2
2
,
1
−
2
1
,
2
16
O-o 1
.
5(
−
2) 5
.
1(
−
2) 8
.
7(
−
2)
1669.9 2
1
,
2
−
1
0
,
1
16
O-o 3
.
8(
−
2) 1
.
9(
−
1) 7
.
4(
−
1)
1716.8 3
0
,
3
−
2
1
,
2
16
O-o 2
.
1(
−
2) 9
.
6(
−
2) 2
.
1(
−
1)
5
2
,
3
transition which is easily detected from the ground at 22
GHz in a broad variety of sources. The 22 GHz line al-
ways show maser emission and is nearly always associated
with regions of on-going star formation. In such regions
maser action is likely to be observed in several submm
lines. The strong 22 GHz maser emission can be explained
by the collisional pumping of dense neutral gas which has
been heated by shocks, either fast and dissociative (J-type
shocks; Elitzur et al. 1989) or slower and non-dissociative
(C-type shocks; Melnick et al. 1993) which can heat the
gas up to about 1000 K.
Following the results of Neufeld & Melnick (1991) we
expect rather strong maser emission from about a dozen of
lines in the frequency range 500 to 1500 GHz which is ac-
cessible to HIFI. The opacities of these lines depend on the
cloud geometry, the collision rates and the volumic den-
sity. In order to be independent both of the geometry and
the gas density we scale the submm transition opacities to
that of the 22 GHz transition. Using Neufeld & Melnick’s
model we select 7 submm transitions that are at least 0.1
to 0.2 times the 22 GHz line intensity and fall in the HIFI
receiver tuning range (Table 3). The opacities in the ta-
ble are relative to that at 22 GHz arbitrarily assumed
Line list
Absorption Lines
A first analysis of the gas lying in front of a limited num-
ber of compact HII regions has been made on the basis
of ground-based observations of several transitions of the
OH radical (eg. Wamsley et al. 1986; Baudry et al. 1993).
The data show that the kinetic temperature of the neutral
gas is of order 100 to 200 K. The ammonia observations
made in the hot cores associated with compact HII regions
.
.
Maser Lines
Many ortho and para rotational levels of water lie close
to each other and thus tend to be easily inverted. Ac-
cordingly, if these levels correspond to allowed radiative
transitions they can give rise to maser amplification. The
strongest water maser emission is that of the 6
1
,
6
−
222
to be -10 for a gas at 1000 K; other mm/submm transi-
tions observed from the ground are given in Table 3 for
comparison. Theoretical predictions of Neufeld & Melnick
(1991) show that maser emission is expected from several
transitions under a wide range of astrophysical conditions.
Therefore, FIRST observations of several submm transi-
tions of water combined with ground-state observations
(Table 3) can be used to constrain the physical conditions
in the H
2
O emitting layers. We wish to observe the seven
150 target sources in all tran-
sitions of water mentioned above. Using the goal system
temperatures expected in the various HIFI receiver bands
we believe that
∼
2 hours are required per source. For a
statistical analysis of a meaningful sample of sources taken
from our survey list we grossly estimate that more than
200-300 hours are required to cover all water lines of inter-
est. This can be achieved only if the FIRST Observatory
decides to support KEY PROGRAM observations.
Table 3. Predicted strong maser transitions in shocked regions.
4. CONCLUSIONS
Frequency Transition o/p Relative
(GHz)
Opacity
a
We discussed that water lines are the main gas coolants
in a variety of conditions typical of the gas surround-
ing protostars, both massive and solar type protostars.
For this reason water lines are unique tools to probe this
gas, its thermal, physical and chemical structure. In this
contribution we presented the case for a sistematic study
from the envelopes of low and high mass protostars. We
wish to emphasize once again that
FIRST will be the
onlyinstrumentabletoobservee0cientlyandrou-
tinely water lines from protostars for many years
to come.
Given the large observing time necessary to
gather a meaningful number of lines in a meaningful num-
ber of sources, the proposed program can only be achieved
if the FIRST Observatory decides to endorse
KEYPRO-
GRAMS
.
22.2
6
1
,
6
−
5
2
,
3
o
-10 (ground)
183.3
3
1
,
3
−
2
2
,
0
p
-4.6 (ground)
325.2
5
1
,
5
−
4
2
,
2
p
-4.9 (ground)
439.2
6
4
,
3
−
5
5
,
0
o
-3.6 (ground)
470.9
6
4
,
2
−
5
5
,
1
p
-1.1 (ground)
530.4 14
3
,
12
−
13
4
,
9
o
-1.7
620.7
5
3
,
2
−
4
4
,
1
o
-4.1
906.2
9
2
,
8
−
8
3
,
5
p
-1.2
970.3
5
2
,
4
−
4
3
,
1
p
-6.5
1158.3
6
3
,
4
−
5
4
,
1
o
-5.5
1440.8
7
2
,
6
−
6
3
,
3
p
-2.9
1542.0
6
3
,
3
−
5
4
,
2
p
-1.7
a
The 22 GHz opacity is arbitrary.
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2
O maser sources are selected according to their total
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−
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. The initial survey list includes about 150
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regions which can sometimes be well separated by the
FIRST-HIFI beam.) Our source list will be further refined
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Time estimate
At the moment it is diAcult to make an exact estimate
of the time required to observe with the HIFI available
µ
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spectrometers a total of
∼
lines in Table 3 not accessible from the ground. They trace
the densest gas layers and we expect that the ratio of some
of these lines together with ground-based observations will
give an estimate of the gas temperature.
Target list
We propose to survey all major HII regions also known
as bright IRAS sources. We concentrate our selection on
sources which have been detected in the 22 GHz line with
the Medicina 32-m survey (Palagi et al. 1993; Brand et al.
199; Codella et al. 1994). The IRAS Point Source Cat-
alogue is used to select (somewhat arbitrarily) objects
brighter than 1000 Jy at 60 and 100
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