L’eau dans le Milieu Insterstellaire Charlotte Vastel
IRAP, octobre 2016
L’eau sur Terre Près de 70 % de la surface de la Terre est recouverte d’eau. Néanmoins, 99 % de notre Terre (en masse) est sèche...
Ligne des glaces: marque la frontière séparant la composante gazeuse des molécules protosolaires associées à C, N et O de leur composante solide (au delà de la limite). Elle marque la séparation entre deux types distincts de planètes : les planètes telluriques et les 2 géantes gazeuses.
Origine de l’eau sur Terre Différents scénarios: 1) Héritage du milieu interstellaire 2) Synthèse lors de la formation des planètes du système solaire 3) Un peu des 2?
Intérêt astrophysique de l’eau ? espèce abondante origine de l’eau sur Terre ? origine de l’eau sur les autres planètes ? chimie de la vie fortement liée à l’eau chimie de l’oxygène agent de refoidissement
Observatoire Spatial Herschel : premières observations “complètes” de l’eau
Water observations with Herschel/HIFI
Ortho
Para
Water transitions available to Herschel/HIFI. Bands 1-5 covers the frequency range 472-1280 GHz; Band 6 covers 1400-1904 GHz; Gap 5-6 is the frequency range between Band 5 and Band 6.
Origin of water in Solar System objects ‣ ‣
Where does the water contained in comets and asteroids come from? How and when did this water form?
Use of water deuterium fractionation (HDO/H2O, D2O/HDO, D2O/H2O)
Star formation
Prestellar core
Class 0 protostar
Class I protostar
Spitzer Science Center IR Compendium
Class II protostar
Protoplanetary disk
Formation in the protoplanetary disk (ion-molecule reactions) ? OR Earlier formation in the molecular cloud or in the protostellar envelope? Vastel et al. 2012; Coutens et al. 2012, 2013a, 2013b, 2014a, 2014b; Taquet et al. 2013; Persson et al. 2014 etc....
Isotopic fractionation: Water
Rosetta
Comets are among the most primitive bodies left from the planetesimal building stage of the Solar Nebula Altwegg et al. 2015
3
He/4He measurements in the solar wind combined with solar models.
Lis et al. 2013
These comets may have originated from radii near the gas giants and subsequently ejected to the Oort cloud (>5,000! au).
7
These comets are believed to originate from the Kuiper belt, which exists beyond the orbits of the giant planets at radii between 30 and 50! AU potential source of water on the Earth! NASA/FUSE/Lynette Cook Hartogh et al. 2011, Nature
Isotopic fractionation: Water
Rosetta
Comets are among the most primitive bodies left from the planetesimal building stage of the Solar Nebula Altwegg et al. 2015
3
He/4He measurements in the solar wind combined with solar models.
Lis et al. 2013
These comets may have originated from radii near the gas giants and subsequently ejected to the Oort cloud (>5,000! au).
8
These comets are believed to originate from the Kuiper belt, which exists beyond the orbits of the giant planets at radii between 30 and 50! AU potential source of water on the Earth! NASA/FUSE/Lynette Cook Hartogh et al. 2011, Nature
Kuiper belt (30-50 UA) Asteroids belt (2-5 UA): carbonaceous chondrites comets (formation in the SS, in the coldest regions, 4.6 billions years ago)
9
the final D/H of water ice after 1 million years of chemical evolution, i.e., the approximate lifetime of the gas-rich disk and, correspondingly, the duration over which the disk is able to build up deuterated water (see Fig. 3). The ions are most abundant in the upper, x-ray–dominated surface layers, whose temperatures are too warm for substantial deuterium enrichment in ½D=H"H þ . 3 The bulk ice mass is closest to the cold midplane, where only a meager amount (per unit volume) of H3þ and H2D+ remain in the gas, a consequence of low ionization rates and high densities.
however, the water is considerably less enriched than the ions. Over the lifetime of the disk, the gas-phase and grain-surface chemical pathways do not produce deuterated water ices in substantial quantities. Water production is low because of a combination of (i) a lack of sufficient ionization to maintain large amounts of H2D+ in the gas and (ii) a lack of atomic oxygen in the gas, locked up in ices. We do find a superdeuterated layer of water at the disk surface (½D=H"H2 O ¼ 5 % 10−3 ). This layer is a direct consequence of selective self-shielding of HD relative to H2, which leads to an overabundance
Origin of water in Solar System objects Theoretical study: Cleeves et al. 2014
•
Aim: Identifying the source of Earth’s (and other solar system objects) water
•
Test with a chemical network if it is possible to reach the terrestrial HDO/H2O ratio in a disk using an initial (D/H)H2 = 2 × 10-5 (solar nebula value)
• Results: The ion-driven deuterium
pathways are inefficient in the disk. A part of interstellar ice survived the formation of solar system and was incorporated into planetesimal bodies.
Fig. 3. Chemical abundances and column densities for the drivers of the deuterium enrichment, the ions; and the corresponding products, the ices. (Top) Volume densities (cm–3) of all labeled isotopologues for H3+ (left) and water (right) in filled purple contours. Solid contour lines indicate local D/H and are as labeled. The color scale applies to both the left and right halves of the plot. (Insets) Enlarged inner disk with the same axis units. (Bottom) Plots of the vertically integrated D/H ratio of column densities versus radius.The bulk gas (protosolar) value is labeled by the red dashed line, and Earth’s ocean value (VSMOW) is labeled with the black dotted line.
“If the solar system’s formation was typical, abundant interstellar ices are available to all nascent planetary systems.” 1592
26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
disk pres Wh ½D= D/H alon bule tica deu Mor that wate ing deu plan war repr dial or s war equ hot izat cess term and ices form tem from we ated mat syst H⨀) ½D= ISM inte for form valu disk com wate valu to 5 com syste imp
How does the water form? Ion Molecule
High-T
EA [K] ~10 000
Warning: Diffuse vs Dense
O+
+
O
H
H2
H+3 +
OH
high abundance
H2
H3O+ low electron abundance in cold clouds
~2 000
Surface H hv H2
OH
s-O s-O2 s-O3
H hv H2
H, H2 hv O, OH
H2
H2O+
~3 000
ee-
H3+
HCO+ e-
hv T
H2O
hv
Gas Phase
s-H2O Grain Surface
is established High D/H three body pr High D/H are found in p Low D/H planes of the
Under mos densities are tablished. A istry out of eq such as the up these conditio by the kinetic ous species in to water form van Dishoeck et al. 2013routes PPVI domin 2.4.2.
The balance between formation and destruction of water in dense clouds leads to a prediction of a fractional abundance of water of 10-8 to a few 10-7 in pure gas-phase models (Smith et al. 2004). This is consistent with observations of diffuse and
Low t
Fig. 2.— Summary of the main gas-phase and solid-state In diffuse chemical reactions leading to the formation and destruction sities less tha translucent clouds but considerably greater than what is observed in cold dense clouds. of H the O water under non-equilibrium conditions. Three different K,are In dense clouds, the bulk of 2 is in the form of ice at levels of H O ice/H (~10 ). Such high ice abundances too water is fo chemical regimes canproduced be distinguished: (i) (Lee ion-molecule large to result from freeze-out of gasphase water by ion-molecule reactions et al. 1996) so grain actions surface (e.g., reactions must surelychemistry, happen. which dominates gas-phase chemistry at low 2
2
-4
How does the water form?
Non-thermal processes: CR induced FUV photo-desorption (10-3) FUV starlight chemical desorption (1% efficiency or more?)
}
Chemical processing
: For water a fraction is released in the gas-phase and the rest is photodissociated and formed back to water.
Thermal processes (sublimation): Heating
adapted from Burke & Brown, 2010, PCCP
Desorption of water ice in the ISM Fraser et al. 2001 No significant differences between: the desorption of H2O from H2O layers and the desorption of H2O from the underlying substrate. “volcano desorption”
In ISM regions where relatively thick ice layers are formed, or the grain surface is hydrophobic, the underlying structure of the cosmic grain is not significant. Elsewhere, the grain material may influence the H2O desorption mechanism.
However, no detection of HDO ice in the cold ISM! HDO/H2O 100
K): the D2O/HDO reflects the ratio in the ices but HDO/H2O is diluted by additional formation of H2O vapor. • gradient of water D/H ratios in the grain mantles (Taquet et al. 2014, Furuya et al. 2016) Molecular cloud
Prestellar envelope
Warm inner regions
Cold outer envelope
hν
hν
hν
hν
high water D/H ratio
T > 100 K
low water D/H ratio
Decrease of the water D/H ratios from the cold outer regions to the warm inner regions
Furuya et al. 2016
CO depletion
ortho-H2D+ + ortho-H2
H3+ + HD
A high o/p H2 is a killer for D!
Interferometric studies Coutens et al. 2014
IRAS 2
Herschel/HIFI: Warm water H218O emission on scales comparable to the protostellar disk, and extended outflow emission. Compact emission of D2O and HDO in the warm inner region of IRAS2. D2O/HDO 7 times higher than HDO/H2O. Natural consequence of chemical evolution in the early cold stages of low-mass star formation: 1) majority of oxygen is locked up in water ice and other molecules in molecular clouds, where water deuteration is not efficient, and 2) water ice formation continues with much reduced efficiency in cold prestellar/protostellar cores, where deuteration processes are highly enhanced due to the drop of the ortho- para ratio of H2, the weaker UV radiation field, etc. (Furuya et al. 2016)
The majority of HDO and D2O ices are likely formed in cold prestellar/protostellar cores rather than in molecular clouds, where the majority of H2O ice is formed
What about prestellar cores?
Gravitational collapse t=0
Molecular cloud Prestellar core
protostar phase
L1544: A prototype for prestellar core studies Many first detections: - H2D+ (Caselli et al. 2004) - H2O (Caselli et al. 2010, 2012) - c-C3D2 (Spezzano et al. 2013) - COMs (Vastel et al. 2014) - CH2CN HFS (Vastel et al. 2015)
12 10
10
8 6
5
4 2
0 0
2000
4000 6000 radius (AU)
0 8000 10000
temperature (K)
Pre-stellar cores provide the original reservoir of material from which future planetary systems are built.
n(H2) x 105 cm-3
d ~ 140 pc Taurus molecular complex
impact parameter (arcseconds) 0 20 40 60 15
Water in the prestellar core L1544 Caselli et al. 2012 First detection of water in a cold dense prestellar core. The foreground cloud material, while collapsing, is absorbing the high-velocity portion of the line profile, producing a brighter blue peak. In the presence of absorption against the continuum background emission, this is equivalent to an inverse P-Cygni profile, as observed in L1544. The water blue emission feature reveals the inner backside of the core, which is approaching the cloud center.
color 1.3 mm
Gravitational contraction motions broaden the water line and allow the blueshifted emission to avoid the foreground “water absorption screen”.
N2H+ (1-0) N2D+ (2-1) H2D+ (110-111)
Where does the water come from?
Red and green curves: models without the cosmic-ray-induced FUV field. Blue curve: simplified model with the cosmic-ray FUV field. Black curve: fractional abundance of water ice (divided by 10000) calculated with the full chemical code, inclusive of the cosmic-ray FUV field.
FUV produced by CR
Hollenbach 2009 10-7
FUV photodesorption of H2O ice
little photodesorption O freeze out
x(H2O)
FUV
10-8
FUV photodissociation H2O gas photo dissociation layer
10-9 1021
H2O ice
photodesorption layer
Nmax1022
freeze-out layer
1023
Caselli et al. 2012
IS FUV
More radiative transfer static envelope infall region -V
Line of sight
+V
The water line is tracing the gravitational
contraction of gas at densities larger than 106 cm−3 , which resides within a radius of 1000 AU.
HIFI / 556.9361 GHz Observation Model from Keto et al. 2014 Best Model
0.14 0.12
The water blue emission feature reveals the inner backside of the core, which is approaching the cloud center.
TMB in K
0.1 0.08 0.06
Gravitational contraction motions broaden the water line and allow the blueshifted emission to avoid the foreground “water absorption screen”.
0.04 0.02 0 -0.02 5
6
-v
7
+v
VLSR in km.s
Blue: Toward the observer 0.1 1
8
9
10
-1
Red: Away from714 the observer 100
Radial distance (arcsec) 10
10
-7
Profile from Keto et al. 2014 Best Ad hoc profile
10-7
Quénard, Taquet, Vastel et al. 2015, A&A
Deuterated water in prestellar cores 0.2 H2O HIFI HDO APEX
0.15
LIME
0.15 T MB in K
0.1 0.1
T mb in K
0.05
APEX / 464.9245 GHz
0.05 0 -0.05 -0.1
0
-0.15
5
6
7
8
V LSR in km.s -1
-0.05
9
10
0.02 0.06
-0.1
APEX 464.9245 GHz
4
5
6
7
V LSR in km.s-1
8
9
10
T MB in K
0.05
APEX 893.6387 GHz
0.015
0.04 0.01
0.03 0.02
0.005
0.01 0
Quénard, Taquet, Vastel et al. 2015, A&A
6.5
7
7.5
V LSR in km.s
-1
8
0
8.5
6
Radial distance (arcsec)
10-7
0.1
10-8
1
10
6.5
7
7.5
V LSR in km.s -1
100
8
8.5
714 10-7
[HDO]/[H2 O] variable
10-8
[HDO]/[H2 O] = 1 [HDO]/[H2 O] = 0.3
10-9
10-10
[HDO]/[H2 O] = 0.1 [HDO]/[H2 O] = 0.03
10-10
10-11
[HDO]/[H2 O] = 0.01
10-11
10 Abundance
Detection limits! Need of a detailed benchmark among different radiative transfer codes for this particular problem of water in prestellar cores.
6
-9
10-12
10-12
10-13
10-13
10-14
10-14
101
102
103 Radial distance (AU)
104
105
What about high-mass star forming regions?
W49N
SgrA*
W31C W28A W51 W33A G34 DR21(OH)
Even worse... complexity + less spatial resolution
W51-core: the most distant prestellar core? 30m and Herschel pointed position
IRS1
e1
e2
towards the telescope
Large scale
IRS2
high veloc
ity filamen t ([62-70]
km/s)
40”
W51 molecular complex: d~5 kpc Mookerjea, Vastel et al. 2013
Large scale
filament ([
62-70] km
/s)
30m and Herschel pointed position
d = 5-6 kpc
Vastel et al. 2016
towards the telescope
40”
W51-core: the most distant prestellar core?
Herschel position
HDO/H2O=0.001 DCO+/HCO+=0.2 DNC/HNC=0.14 T~10K nH2~2.5 104 cm-3
Dark cloud conditions (TMC1) NH2D Result from the collision of the filament with W51
IRAM/NOEMA
Vastel+2016
From prestellar, to protostar and planet
Relative abundances of HDO and H2O Comets
Protostars
Prestellar
Earth’s oceans
Cosmic D/H
(2014) Possible mechanism for an increased ratio of HDO/H2O in the gas phase (compared with that Persson+ initially in the ice): prolonged photoprocessing of the outer layers of icy mantles. Photo-excitation of HDO leads preferentially to desorption of H (Arasa et al. 2015). The enrichment of the ice in D atoms due to photo-desorption can, over time, lead to an enhanced HDO/H2O ratio in the ice, and, when photo-desorbed, also in the gas.
Thanks for your attention!
