219
HIFI SURVEY OF WATER LINES FROM PROTOSTARS
C. Ceccarelli1, A. Baudry1, E. Caux2, X. Tielens3, S. Bontemps1, J. Braine1, A. Castets1, M. Giard2,
F. Helmich3, F. Herpin1, T. Jacq1, C. Joblin2, S. Maret2, I. Ristorcelli2, and C. Vastel2
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 the warm inner regions of the envelopes surrounding low
luminosity protostars (Ceccarelli, Hollenbach & Tielens
Since water is very abundant and easily excited in the
1996; Doty & Neufeld 1997). These early predictions were
circumstellar environments, water lines are an extremely
confirmed by the observations of strong water emission
useful tool to probe the innermost regions of the envelopes
in several low mass protostars carried out with the two
surrounding low mass protostars as well as the immediate
spectrometers on board the Infrared Space Observatory
environment of compact and ultra-compact HII regions.
(ISO) (e.g. Ceccarelli et al. 1999). The detailed analy-
In this contribution we discuss the importance to carry
sis of the water line spectrum of the solar type protostar
out a systematic survey of selected water lines in low and
IRAS16293-2422 allowed to reconstruct the physical struc-
high mass protostars with the high resolution spectrom-
ture of its envelope and to estimate the two key parameters
eter HIFI on board FIRST. Given the large amount of
of the protostar: the mass of the central forming star and
observing time necessary to survey a meaningful number
its mass accretion rate (Ceccarelli et al. 2000). In addition,
of lines in a meaningful statistical sample, this survey can
the observed water lines enabled us to estimate the water
only be achieved by means of a KEY PROGRAM.
abundance in the cold outer envelope and in the warm
innermost regions enriched in water vapour, both because
Key words: Stars: formation Missions: FIRST
of the evaporation of the water ice previously stored in
the cold grain mantles and because of efficient chemical
reactions which lock most of the gaseous oxygen into wa-
ter molecules. What is important to note here is that the
1. Introduction
model, corroborated by the H2O line observations, pre-
Oxygen is the third most abundant element in the Uni-
dicts the existence of a misty and warm region at about
verse, after Hydrogen and Helium. In cold molecular clouds
150 AU where about 0.035 M of water vapour collapses
it is mainly in the form of gaseous CO, O and H2O, (e.g.
towards the center. Once entered into the protoplanetary
van Dishoeck & Blake 1998; Caux et al. 1999) and in the
disk, the water vapour may condense onto the cold grains,
form of iced water coated around the grains (e.g. Tielens
which then aggregate to form icy planetesimals imprinted
1987). However, in star forming regions most of the oxy-
with this first collapse phase memory (Chick & Cassen
gen can be found in water, because of the evaporation of
1997).
the grain ice mantles and/or of the formation of water in
Given the relatively low sensitivity, low spatial reso-
the gas phase via endothermic reactions (e.g. Kaufman &
lution and low spectral resolution, the ISO observations
Neufeld 1996; Ceccarelli, Hollenbach & Tielens 1996). Be-
had some obvious limitations, which will be certainly over-
cause of its high abundance and easy excitation in warm
come by FIRST. With its higher sensitivity, and spatial
interstellar and circumstellar environments water is a pow-
and spectral resolutions, FIRST will be able:
erful tool to probe astrophysical conditions in a broad va-
a) to increase the sample of solar type protostars where
riety of sources, from protostars to molecular shocks. In
this contribution we focus on the case for systematic obser- the study of water line emission is possible;
b) to disentangle the water emission associated with the
vations of water lines in low and high mass protostars. We
specifically address the unique capabilities of HIFI in ob- infalling gas against the emission due to shocked ma-
terial of the outflowing gas;
taining high spectral and spatial resolutions observations
c) to resolve the line emission thus making possible kine-
of the submm rotational transitions of water to study the
immediate environment of solar type protostars and com- matical studies: note that since high energy lying water
lines originate in the innermost regions, where the in-
pact or ultra-compact HII regions.
fall velocity is the highest, they can be resolved by the
HIFI spectrometer and used to probe the infall;
2. Low mass protostars
d) and finally, to measure water abundances in the cold
Water lines have been predicted to be the most abun- envelopes with a sensitivity at least an order of mag-
dant oxygen bearing molecule and major gas coolants of nitude better than SWAS.
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
220 C. Ceccarelli et al.
FIRST will be the only instrument able to observe effi- 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
ciently and routinely water lines from solar type protostars
pc (more details in Ceccarelli et al. 2000).
for many years to come.
In conclusion, the water lines observable with FIRST
Frequency Transition o/p Flux
are of paramount importance to derive key parameters for
the process of a solar type protostar:
(GHz) erg s-1 cm-2
the mass of the central forming stars and their accre-
tion rates; 556.9 11,0 - 10,1 o 4.2E-13
752.0 21,1 - 20,2 p 1.5E-13
the water abundance in the cold gas of the outer en-
916.1 42,2 - 33,1 p 2.2E-14
velope and
970.3 52,4 - 43,1 p 1.7E-14
the enrichment of water of the innermost regions, which
1097.4 31,2 - 30,3 o 1.3E-13
may ultimately form planetary systems.
1153.1 31,2 - 22,1 o 2.1E-13
The knowledge of these parameters in a statistically mean-
1158.3 63,4 - 54,1 o 1.2E-14
ingful and well selected sample of solar type protostars
1162.9 32,1 - 31,2 o 1.1E-13
would be invaluable to understand the processes that lead 1228.8 22,0 - 21,1 p 1.0E-13
1296.4 82,7 - 73,4 o 3.6E-15
to the formation of stars and planetary systems.
1440.9 72,6 - 63,3 p 5.5E-15
1602.2 41,3 - 40,4 p 8.0E-14
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.
process of the low mass star formations, such as for is-
In order to make realistic predictions we ran a grid of
tance the sound speed in the parent cloud (e.g. Shu et al.
models for different protostar luminosities, masses, mass
1987).
accretion rates, water abundances etc. The selected lines
(Tab. 1) represent the result of this modeling effort. In
Time estimate
addition to H16O lines we propose to survey also two key
2 To calculate the observing times we request a 2Ă detec-
H18O lines, which will allow to derive in detail the col-
2 tion level over three channels (each of 1/3 of the predicted
umn densities of the observed lines and therefore to bet-
line width). Using the HIFI available spectrometers we
ter reconstruct the envelope structure. Finally three HDO
propose to observe the lines In Table 1. The system tem-
lines will also be surveyed, appropriately selected to de-
peratures used to give the estimate of the observing time
rive the water deuteration through the envelope. Although
are roughly those given as the baseline on page 48 of
it is in principle possible to observe HDO from ground,
the Scientific and Technical case for HIFI (Part I). Based
the very few observations available in literature show that
on the mentioned model predictions and such observing
these observations are very difficult. On the other hand
time estimates to observe a meaningful set of water lines
it is now clear that in solar type protostars molecules like
in sources whose luminosity is about 1 L will take around
H2CO and NH3 present extremely large degrees of deuter-
four hours per source (no overheads are taken into account
ation, more than 10% being in the deuterated forms of
yet). To survey the proposed star forming regions will need
these molecules (Ceccarelli et al. 1998; Loinard et al. 2000;
therefore about 300 hours of observations (with no over-
Roueff et al 2000). It will therefeore be extremely impor-
heads), which makes this study only possible via a KEY
tant to study the deuterated form of H2O in the same
PROGRAM.
objects to understand the route of deuteration of these
molecules.
3. High mass protostars
Target list
It is proposed to survey some of the nearby low mass In the second part of this contribution we address the
star forming regions: Taurus, Á Ophiuchus, Perseus, Ser- question of the complex environment of embedded mas-
pens, Orion and Chameleon are obvious examples. Over- sive O- or B-type stars which have not yet fully dispersed
all these complexes contain about 80 embedded low lu- their natal cloud material. We specifically wish to use the
minosity protostars, two thirds of them with luminosities high spectral and spatial resolutions achieved with HIFI
between 1 and 3 L , and the remaining with higher lu- in the submm rotational transitions of water to study the
minosities. Comparison of objects within the same region immediate environment of compact or ultra-compact HII
have been already shown to be extremely useful to draw regions. These regions are detected throughout the Galaxy
evolutionary pictures (e.g. Saraceno et al. 1996; Bontemps in both the radio and far infrared domains. Several of
et al. 1996). Comparison between different regions high- them have been mapped with radio interferometers and
lights if and which macroscopic parameters enter in the are known as luminous IRAS sources. Their total lumi-
HIFI Survey of Water Lines from Protostars 221
nosity is of the order of 103 L or more and may reach confirm this result and suggest that velocity and tempera-
105 L in the extreme case of W3(OH). They are often ture gradients can be present in the cores (eg. Cesaroni et
associated with massive molecular clouds and masers and al. 1998). Besides, ISO-SWS observations already showed
they sometimes exhibit pronounced molecular outflows. the potentiality of this technique (e.g. van Dishoeck &
The spatial resolution of FIRST, of the order of 15 to Helmich 1996; Helmich et al. 1996; van Dishoeck et al.
20 -or less for the highest frequencies of water-, does not 1998; Wright et al. 2000). To identify the strongest H2O
permit mapping the most compact HII regions although it absorption lines (Table 2) in the neutral gas in front of the
helps to discriminate the embedded object from the more compact HII regions we used the model results by Doty
extended molecular environment. However, depending on & Neufeld (1997) who explicitely computed the water line
the target and the selected transition we expect to ob- spectra of massive protostars.
serve both absorption and emission in several transitions
of water, including maser emission, from the gas layers in
Table 2. Predicted water absorption lines for a cloud of mass
the neighbourhood of the HII region (see (i) and (ii) be-
100 M , illuminated by a central source of luminosity vary-
low). In HII regions most of the oxygen is found in water
ing beteewn 103 and 105 L , as computed by Doty & Neufeld
because icy grain mantles have been evaporated or be-
(1997).
cause endothermic gas phase reactions dominate (see the
Introduction). Combined with easier excitation in warm
16
regions water is thus an efficient coolant of the neutral
Freq. Transition O/18O Line Luminosities (L )
material surrounding the ionized gas associated with the
(GHz) o/p 103L 104L 105L
HII region. 1101.7 11,1 - 00,0 18O-p 4.1(-3) 9.3(-3) 2.5(-2)
1113.3 11,1 - 00,0 16O-p 1.5(-2) 5.3(-2) 2.5(-2)
(i) Absorption lines are most interesting because they
1655.9 21,2 - 10,1 18O-o 1.5(-2) 5.5(-2) 1.4(-1)
probe the compressed neutral regions lying along the
1661.0 22,1 - 21,2 16O-o 1.5(-2) 5.1(-2) 8.7(-2)
line of sight to the HII regions with excitation less
1669.9 21,2 - 10,1 16O-o 3.8(-2) 1.9(-1) 7.4(-1)
than the background emission. These lines thus nicely
1716.8 30,3 - 21,2 16O-o 2.1(-2) 9.6(-2) 2.1(-1)
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 Maser Lines
order 100 K or above. Detectability of this gas de- Many ortho and para rotational levels of water lie close
pends on the filling factor of the FIRST telescope beam to each other and thus tend to be easily inverted. Ac-
and does not specifically probe the gas immediately cordingly, if these levels correspond to allowed radiative
against the HII region. On the other hand, the anoma- transitions they can give rise to maser amplification. The
lously excited lines leading to strong maser emission strongest water maser emission is that of the 61,6 - 52,3
because of shocks and collisional and infrared pumping transition which is easily detected from the ground at 22
of water could also be detected despite beam dilution. GHz in a broad variety of sources. The 22 GHz line al-
These masing transitions trace the warmest and dens- ways show maser emission and is nearly always associated
est pockets of the gas or just trace the shocked regions; with regions of on-going star formation. In such regions
they allow us to investigate the small-scale clumpiness maser action is likely to be observed in several submm
of the outer neutral gas layers against the HII region. lines. The strong 22 GHz maser emission can be explained
In conclusion, by surveying a number of submm transi- by the collisional pumping of dense neutral gas which has
been heated by shocks, either fast and dissociative (J-type
tions of water in a large sample of sources throughout the
Galaxy we expect to derive a statistical view of the com- shocks; Elitzur et al. 1989) or slower and non-dissociative
(C-type shocks; Melnick et al. 1993) which can heat the
plex physical conditions prevailing in the gas surrounding
gas up to about 1000 K.
embedded massive stars. We thus hope to bring new light
on the early evolutionary stages of embedded massive ob- Following the results of Neufeld & Melnick (1991) we
jects. expect rather strong maser emission from about a dozen of
lines in the frequency range 500 to 1500 GHz which is ac-
Line list cessible to HIFI. The opacities of these lines depend on the
Absorption Lines cloud geometry, the collision rates and the volumic den-
A first analysis of the gas lying in front of a limited num- sity. In order to be independent both of the geometry and
ber of compact HII regions has been made on the basis the gas density we scale the submm transition opacities to
of ground-based observations of several transitions of the that of the 22 GHz transition. Using Neufeld & Melnick s
OH radical (eg. Wamsley et al. 1986; Baudry et al. 1993). model we select 7 submm transitions that are at least 0.1
The data show that the kinetic temperature of the neutral to 0.2 times the 22 GHz line intensity and fall in the HIFI
gas is of order 100 to 200 K. The ammonia observations receiver tuning range (Table 3). The opacities in the ta-
made in the hot cores associated with compact HII regions ble are relative to that at 22 GHz arbitrarily assumed
222
to be -10 for a gas at 1000 K; other mm/submm transi- spectrometers a total of <" 150 target sources in all tran-
tions observed from the ground are given in Table 3 for sitions of water mentioned above. Using the goal system
comparison. Theoretical predictions of Neufeld & Melnick temperatures expected in the various HIFI receiver bands
(1991) show that maser emission is expected from several we believe that <" 2 hours are required per source. For a
transitions under a wide range of astrophysical conditions. statistical analysis of a meaningful sample of sources taken
Therefore, FIRST observations of several submm transi- from our survey list we grossly estimate that more than
tions of water combined with ground-state observations 200-300 hours are required to cover all water lines of inter-
(Table 3) can be used to constrain the physical conditions est. This can be achieved only if the FIRST Observatory
in the H2O emitting layers. We wish to observe the seven decides to support KEY PROGRAM observations.
4. CONCLUSIONS
Table 3. Predicted strong maser transitions in shocked regions.
We discussed that water lines are the main gas coolants
Frequency Transition o/p Relative
in a variety of conditions typical of the gas surround-
(GHz) Opacitya
ing protostars, both massive and solar type protostars.
For this reason water lines are unique tools to probe this
22.2 61,6 - 52,3 o -10 (ground) gas, its thermal, physical and chemical structure. In this
183.3 31,3 - 22,0 p -4.6 (ground)
contribution we presented the case for a sistematic study
325.2 51,5 - 42,2 p -4.9 (ground)
from the envelopes of low and high mass protostars. We
439.2 64,3 - 55,0 o -3.6 (ground)
wish to emphasize once again that FIRST will be the
470.9 64,2 - 55,1 p -1.1 (ground)
only instrument able to observe efficiently and rou-
530.4 143,12 - 134,9 o-1.7
tinely water lines from protostars for many years
620.7 53,2 - 44,1 o-4.1
to come. Given the large observing time necessary to
906.2 92,8 - 83,5 p-1.2
gather a meaningful number of lines in a meaningful num-
970.3 52,4 - 43,1 p-6.5
ber of sources, the proposed program can only be achieved
1158.3 63,4 - 54,1 o-5.5
if the FIRST Observatory decides to endorse KEY PRO-
1440.8 72,6 - 63,3 p-2.9
GRAMS.
1542.0 63,3 - 54,2 p-1.7
a
The 22 GHz opacity is arbitrary.
References
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Bontemps S. et al. 1996, A&A 311, 858
lines in Table 3 not accessible from the ground. They trace
Caux E. et al. 1999, A&A 347, L1
the densest gas layers and we expect that the ratio of some
Ceccarelli C., Hollenbach D. & Tielens A. 1996, ApJ 471, 400
of these lines together with ground-based observations will
Ceccarelli C. et al. 1998, A&A, 338, L43
give an estimate of the gas temperature.
Ceccarelli C. et al. 1999, A&A, 342, L21
Target list
Ceccarelli C.et al. 2000, A&A 355, 1129
We propose to survey all major HII regions also known
Cernicharo J. et al. 1999, 520, L131
as bright IRAS sources. We concentrate our selection on Cesaroni R. et al. 1998, A&A 331, 709
Chick K.M. & Cassen P. 1997, ApJ 477, 398
sources which have been detected in the 22 GHz line with
Codella C. et al. 1994, A&A 291, 261
the Medicina 32-m survey (Palagi et al. 1993; Brand et al.
Doty S.D. & Neufeld D.A. 1997, ApJ 489, 122
199; Codella et al. 1994). The IRAS Point Source Cat-
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alogue is used to select (somewhat arbitrarily) objects
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H2O maser sources are selected according to their total
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integrated flux; we take here sources brighter than about
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100 Jy km s-1. The initial survey list includes about 150
Neufeld D. & Melnick G. 1991 ApJ 368, 215
objects. (It includes some major complexes such as W49,
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W51 or W75; these regions contain several individual HII
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regions which can sometimes be well separated by the Saraceno P. et al. 1996, A&A 309, 827
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Time estimate
Wright, C. M et al. 2000, A&A 358, 689
At the moment it is difficult to make an exact estimate
of the time required to observe with the HIFI available
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