H 2 O and CO Ices in Protostellar Environments Recent Keck Telescope Results. Adwin Boogert California Inst. of Technology. Interstellar Ices-I

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1 Interstellar Ices-1 H 2 O and CO Ices in Protostellar Environments Recent Keck Telescope Results Adwin Boogert California Inst. of Technology Jan/2005 Interstellar Ices-I

2 Contents Infrared spectra of highly obscured (proto)stars: overall spectra in different environments: quiescent clouds, high mass protostars, low mass protostars, edge-on disks Where are the ices? Ice formation threshold. Mantle thickness. Solid H 2 O and CO vibrational modes. Column densities. H 2 O amorphous/crystalline phases. Time scales. Directly observing ices in protostellar disks. Polar versus apolar ices Keck telescope results: The solid 12 CO and 13 CO bands Grain shape and size effects. General conclusions interstellar ice or dirty ice : any frozen volatile, e.g. H 2 O, H 2 O mixtures, pure CO Jan/2005 Interstellar Ices-I

3 Infrared Spectra of Highly Obscured (Proto)Stars Ice and dust absorption bands observed against continuum of a star or protostar backgroun Jan/2005 Interstellar Ices-I

4 Infrared Spectra of Highly Obscured (Proto)Stars Ground based telescopes can only observe fraction of ice bands due to absorption in earth's atmosphere. Study of important species (CO 2, CH 4, C-H/C-O bending modes in 5-8 µm region) thus severely hindered; use satellites: Jan/2005 ISO ( ) Interstellar Ices-I

5 Infrared Spectra of Highly Obscured (Proto)Stars Ices formed anywhere T<90 K and extinction large enough Jan/2005 Interstellar Ices-I

6 Infrared Spectra of Highly Obscured (Proto)Stars Single dish versus interferometer observations of CO toward protostar Interferometer filters out extended emission. Jan/2005 Interstellar Ices-I

7 Column Density Ice Formation Threshold Due to grain temperature and interstellar radiation field ices form only if visual extinction (A V ) large enough: the ice formation threshold Taurus cloud: H 2 O ices absent below visual extinction A V ~3 and CO ices below A V ~7. Difference due to lower T sub of CO. Variation between clouds due to different temperature/radiation field H 2 O CO Extinction (A V ) Jan/2005 Interstellar Ices-I

8 Ice Mantle Thickness H 2 O formed by grain surface reactions (Cecilia's lectures), CO formed in gas and inertly condenses on grains. Grain mantle thickness: Mass growth rate: dm/dt=s*π*a 2 *n*<v>*<m> Radius growth rate: da/dt=(dm/dt)/(4*π*a 2 *ρ) da/dt=s*n*<v>*<m>/(4*ρ) Mantle thickness independent of grain radius Dense clouds can have mantles as thick as 0.1 um. Deeply embedded protostars even more. mantle thicker than most grain cores according to MRN grain size distribution n(a)~a -3.5, a min =0.005 um, a max =0.25 um Jan/2005 Interstellar Ices-I

9 Solid H 2 O and CO Vibrational Modes Gas phase CO: ro-vibrational transitions allow ΔJ=1, Δv=1; characteristic P and R branch spectrum. Solid CO: vibrations only giv broader absorption whose wid position and shape is determi by solid state (dipole) interac High resolution required to separate gas and solid bands. [ISO satellite observation of Elias 29 in Rho Oph cloud; Boogert, Tielens, Ceccarelli et al. A&A 360, 683, 2000] Jan/2005 Interstellar Ices-I

10 Solid H 2 O and CO Vibrational Modes Jan/2005 Interstellar Ices-I 1

11 Solid H 2 O and CO Vibrational Modes Jan/2005 Interstellar Ices-I 1

12 Solid H 2 O and CO Vibrational Modes Ice column densities: N=τ peak *FWHM/A lab A lab integrated band stren measured in laboratory A[H 2 O 3 µm]=6.2x10-16 cm/mol. How much ice is there? Order of magnitude in quiescent dense clouds: N(H 2 O-ice)=10 18 cm -2 normalized to N H X(H 2 O-ice)~10-4 ~X(CO-g [For reference: this is ice layer of 0.3 µm at 1 g/cm 3 in Jan/2005 Interstellar Ices-I 1

13 H 2 O Crystallization Interstellar H 2 O ices formed in amorphous phase, as evidenced by prominent 'blue' wing. Crystallization by protostellar heat. [long wavelength wing originates from scattering on large grains and NH 3 :H 2 O complex Crystallization temperature ~120 K in laboratory, but ~70 K in spa due to longer time scales. [Time scale ~exp(-t/e barrier ) (~1 hour in lab, 10 5 yr in Jan/2005 Interstellar Ices-I 1

14 Ices in Circumstellar Disks Boogert, Hogerheijde, & Blake, ApJ 568, 761 (2002) Jan/2005 Interstellar Ices-I 1

15 Direct Observations of Ices in Circumstellar Disks Protostellar disks provide crucial link between evolution of ic from molecular clouds planetary systems (comets). Major difficulty: does line of sight pass throu disk and which part of disk? Disk needs to be edge-on. (Pontoppidan et al. 2005, ApJ, in press see also Jan/2005 Interstellar Ices-I 1

16 Direct Observations of Ices in Circumstellar Disks Disk inclination crucia study of ices (edge-on or gas (face-on). Movie shows 2 µm scattering model of 50 AU disk surrounding 1 M sun protostar (Pontoppidan et al. 2005, ApJ, in pr see also Jan/2005 Interstellar Ices-I 1

17 Ices in Disks Direct observations of ices in disks only possible for edge-on disks (obviously). Difficult, rarely done, and exact ice location often disputed. Few claims were made (Kastner et al. 1995; Shuping et al. 2000; Boogert et al. 2002; Thi et al. 2002) Understanding of ices in disks requires knowledge of disk properties (e.g. inclination) through mm-wave observations. Jan/2005 Interstellar Ices-I 1

18 The Shrinking Disk of L1489 IRS HCO + OVRO/ BIMA: HST 2 µm Class I SED, L=3.7 L sun, M sun sun, M disk disk =0.02 Large flattened structure in millimete continuum+line emission. Continuum and lines cannot be fitted with same inside-out envelope collapse model Instead, velocity structure indicates large 2000 AU radius close to edge- on disk,, in almost Keplerian rotation (v~1/r 0.5 ) with infall component. L1489 IRS represents short lived (<20,000 yr) transition phase in which disk Hogerheijde, shrinks ApJ to few 553, AU (200 TTauri disk? Jan/2005 Interstellar Ices-I 1

19 The Shrinking Disk of L1489 IRS CO Keck/NIRSPEC: Gas phase close-up: To first order, 4.7 µm CO spectra confirm infalling motion of disk. scale height increases more than linear, very flared disk Too much warm gas at high velocity Only partial collapse of disk, e.g. a surface layer and outer parts? Jan/2005 Interstellar Ices-I 1

20 Ices in Disk L1489 IRS Prominent band of solid CO detected toward L1489, originating in large, flaring disk. CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Jan/2005 Interstellar Ices-I 2

21 H 2 O versus CO Physical Characteristics High dipole moment of H 2 O compared with CO results in very different physical and spectral characteristics: Sublimation temperature much higher for H 2 O (90 K vs. 18 K in space) Bands much broader for H 2 O (amorphous) H 2 O/CO mixtures: distinct polar and apolar ices with different H 2 O/CO ratios that can spectroscopically be distinguished and sublimate at different T. Highly relevant for comets as well, as dipole moment of ice determines outgassing behaviour. 'Pockets' of apolar CO may result in sudden sublimation. Jan/2005 Interstellar Ices-I 2

22 Ices in Disk L1489 IRS Prominent band of solid CO detected toward L1489, originating in large, flaring disk. CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: 'polar' H 2 O:CO (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Jan/2005 Interstellar Ices-I 2

23 Ices in Disk L1489 IRS Prominent band of solid CO detected toward L1489, originating in large, flaring disk. CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: 'polar' H 2 O:CO 'apolar' CO 2 :CO or pure CO phase [NEW!] (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Jan/2005 Interstellar Ices-I 2

24 Ices in Disk L1489 IRS Prominent band of solid CO detected toward L1489, originating in large, flaring disk. CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: 'polar' H 2 O:CO 'apolar' CO 2 :CO or pure CO phase [NEW!] 'apolar' pure CO (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Jan/2005 Interstellar Ices-I 2

25 Ice Processing in Disk Are ices in L1489 IRS disk processed? Jan/2005 Interstellar Ices-I 2

26 Ice Processing in Disk Are ices in L1489 IRS disk processed? Empirical answer by comparing CO ice band with established unprocessed line of sight, NGC 7538 : IRS9: (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 2

27 Ice Processing in Disk Are ices in L1489 IRS disk processed? Empirical answer by comparing CO ice band with established unprocessed line of sight, NGC 7538 : IRS9: apolar CO-rich ices evaporate in L1489 IRS disk (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 2

28 Is CO in disk thermally processed? Ice Processing in Disk YES, although disk mid-planes expected to have very large depletion, CO depletion L1489 IRS disk only 7% related to slightly inclined disk (0-30 o ): We see thermally processed ices in disk upper layers. One other disk studied (CRBR2422: also thermal processing selection effect? Has CO ice composition changed by energetic processing? Apolar CO: NO. Evaporation has occurred in L1489 IRS but profile remains the same. No CO 2 formation. Polar CO: NO. Pristine line of sight (NGC 7538 IRS9) and L1489 IRS have the same abundance of polar CO w.r.t. solid H 2 O abundance (8%) Jan/2005 Interstellar Ices-I 2

29 Detection of Solid 13 CO: New Clues to the Nature of Apolar Ices Boogert, Blake, & Tielens, ApJ 577, 271 (2002) Jan/2005 Interstellar Ices-I 2

30 Detection of Solid 13 CO NGC 7538 : IRS9: First detection of solid 13 CO High spectral resolution required! (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 3

31 Detection of Solid 13 CO NGC 7538 : IRS9: First detection of solid 13 CO High spectral resolution required! New insights into nature apolar ices: 13 CO well fitted with pure CO, but 12 CO... (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 3

32 Grain Shape and Size Effects Laboratory and interstellar absorption spectra cannot always be directly compared: Scattering on large (micron sized) grains leads to 3 um red wing (often observed) Surface modes in small grains may lead to large absorption profile variations: Ice refractive index m=n+ik Absorption cross section ellipsoidal grain proportional to (Mie theory): (2nk/L 2 )/[(1/L-1+n 2 - k 2 ) 2 +(2nk) 2 ] Resonance for sphere (L=1/3) occurs at k 2 -n 2 =2, so at large k (=strong transitions): important for pure CO, but not for CO diluted in H 2 O and also not for 13 CO. Jan/2005 Interstellar Ices-I 3

33 Detection of Solid 13 CO NGC 7538 : IRS9: First detection of solid 13 CO High spectral resolution required! New insights into nature apolar ices: 13 CO well fitted with pure CO, but 12 CO......requires ellipsoidal grains (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 3

34 Detection of Solid 13 CO NGC 7538 : IRS9: First detection of solid 13 CO High spectral resolution required! New insights into nature apolar ices: 13 CO well fitted with pure CO, but 12 CO......requires ellipsoidal grains CO and CO 2 not mixed Finally: isotope ratios (Boogert, Blake & Tielens, ApJ 577, 271 (2002)) Jan/2005 Interstellar Ices-I 3

35 General Conclusions Location, location, location: Infrared observations of ices and (sub-)mm observations of gas need to be combined to determine physical conditions and location of ices. Interstellar ices formed amorphous, but crystallize by heat from nearby protostar Both polar and apolar ices are present in the ISM Ices detected in few disks low mass YSOs. So far ices appear to originate in warm upper layers show evaporation of apolar ices. Perhaps selection effect as specific orientation needed to see ices in the first place. No signs energetic processing. Promising start, but thorough study of large sample of objects both in gas and solid state needed to answer question on origin and evolution of solar system ices. Simultaneous observation of 12 CO and 13 CO tells us that: 13 CO isotope powerful tracer of ices. Apolar component likely pure CO grains have CDE (~irregular) shapes 12 / 13 CO ratio Jan/2005 Interstellar Ices-I 3