1 / 17

Observations of deuterated molecules as probes of the earliest stages of star formation.

Observations of deuterated molecules as probes of the earliest stages of star formation. Helen Roberts University of Manchester. Dark Clouds: T ~ 10K H 2 density ~ 10 4 cm -3 Contain large, unsaturated molecules Typical D/H ratios are 1-10%. E 1 ~ 200 K; E 2 ~ 370 K; E 3 ~550 K.

oma
Download Presentation

Observations of deuterated molecules as probes of the earliest stages of star formation.

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Observations of deuterated molecules as probes of the earliest stages of star formation. Helen Roberts University of Manchester

  2. Dark Clouds: • T ~ 10K • H2 density ~ 104cm-3 • Contain large, unsaturated molecules • Typical D/H ratios are 1-10%

  3. E1 ~ 200 K; E2 ~ 370 K; E3 ~550 K HD/H2 ~ 10-5

  4. H3+ HCO+, N2H+ CO, N2 HD H2D+ DCO+, N2D+ E1 ~ 200 K; E2 ~ 370 K; E3 ~550 K

  5. When comparing observations with results from chemical models, molecular D/H ratios can be more useful than absolute abundances. • The predicted abundances are sensitive to the reaction rates used in the model. • The molecular D/H ratios are more sensitive to the physical conditions assumed. Fractional abundances over time from two gas-phase chemical models. In each case T=10K and n(H2)=104cm-3. The filled symbols show results from rate99, the hollow symbols OSU (Roberts et al. 2004).

  6. The D/H ratios, however, all agree to within a factor of three. Several commonly observed species have abundances differing by more than a factor of five.

  7. The reason for this, though, is the underlying assumption in the deuterium chemistry models. • We assume that deuterated species react with the same rates as un-deuterated species. • Where there is uncertainty as to which product the D ends up on, we assume statistical branching ratios. H3+ + CO  HCO+ + H2 k cm3s-1 H2D+ + CO  DCO+ + H21/3k cm3s-1  HCO+ + HD 2/3k cm3s-1 But some experiments suggest that this may not always be true: H3+ + e- H2 + H 1.4e-8 cm3s-1  H + H + H 5.4e-8 cm3s-1 H2D+ + e-  HD + H 2/3 x 1.4e-8 4/5 x 5.4e-8cm3s-1  H2 + D 1/3 x 1.4e-8 1/5 x 5.4e-8cm3s-1  H + H + D 5.4e-8 4.4e-8cm3s-1 Experimental rates at 300K (McCall et al. 2003) Experimental rates from Sundström et al. 1994) Statistical rates

  8. Prestellar cores: • T ~ 8-10 K • Central density ~ 106cm-3 • Heavy depletion of species like CO and CS is observed Observations from Caselli et al. (1999) showing CO depletion across L1544 Image of L1544: Contours show N2H+; colour scale is CCS. • Molecular D/H ratios are enhanced (>10%)

  9. HD,D2,D,H D2,D H2,H HD,H2 D,H e- H2 HD2+ D3+ H3+ H2D+ HD CO,N2,O DCO+,HCO+,N2D+, N2H+,OD+,OH+ HCO+,N2H+,OH+ DCO+,N2D+,OD+ Models which include gas-phase reactions and freeze-out onto grains reproduce the observational result that D/H ratios increase with depletion. H3+ is converted to its deuterated analogues. Multiply deuterated H3+ is more efficient at deuterating other species. Observations of 5 prestellar cores by Bacmann et al. (2000) Prediction from model

  10. HD,D2,D,H D2,D H2,H HD,H2 D,H e- H2 HD2+ D3+ H3+ H2D+ HD • At the very centre of the core, in the last stages before the star forms, we expect all heavy species to be frozen onto grains. • At late times the abundance of H2D+ is similar to HD2+. • D3+ becomes the most abundant deuterated molecule. • The atomic D/H ratio rises to ~0.8, which is important for surface chemistry Results from an `accretion’ model. T=10K; n(H2) = 106 cm-3 (Roberts et al. 2003)

  11. Recent Observations of deuterated H3+: Caselli et al. (2003) detected H2D+ towards L1544. The emission is strong towards the dust peak, and much weaker at the off-peak position. This suggests that H2D+ is most abundant in the core centre. Vastel et al. (2004) made the first detection of HD2+ last year. The abundance of HD2+ appears to be similar to that of H2D+.

  12. HD,D2,D,H D2,D H2,H HD,H2 D,H e- H2 HD2+ D3+ H3+ H2D+ HD Once all molecules heavier than H, He and D have frozen out, the relative abundances of H3+ and its analogues depend on the electron abundance. As H2D+ and HD2+ can be observed via their rotational spectra, they can be used to probe the ionisation fraction in the last stages before a star forms. Prestellar core: L1544 • These molecules may also be useful probes of the conditions at the midplane of proto-planetary disks, where densities are also high, and heavy species are depleted. • Ceccarelli et al. (2004) recently detected H2D+ in two disks, and estimate that the fractional electron abundance is a few x10-10 Protoplanetary disks Figure from Ceccarelli et al. (2004). Showing a simple model for H2D+ abundance as a function of electron abundance.

  13. Protostellar Sources: • These are complex regions, containing jets and outflows, and having temperature and density variations. • Both high and low-mass star protostars are observed to have `hot core’ regions, where the gas has warmed up enough to evaporate grain mantles. • These hot core regions have enhanced abundances of saturated molecules (e.g. H2O, CH3OH, H2S), indicative of surface chemistry. • Deuterated molecular ions tend to have lower fractionation (indicates higher temperature). • Surface species can have very high D/H ratios. This depends on the atomic D/H ratio in the gas when they formed.

  14. IRAS 16293 is one of the best studied low-mass star forming regions and has high deuterium fractionation. • Parise et al. (2003, 2004) have detected 4 isotopomers of deuterated methanol there. • Results for three of the methanol species (CH2DOH, CHD2OH and CD3OH) can be produced with an atomic D/H ratio of 0.1-0.2. • CH3OD, however, has a lower fractionation than we would expect. • Could this be due to reactions in the gas-phase after evaporation? Observations from Parise et al. compared with model results from Stantcheva et al. (2003), showing methanol fractionation on the grains vs. the D/H ratio in the accreting gas.

  15. The models assume that methyl groups which are present on both products and reactants are unaffected by the reaction (Osamura et al. 2004). So for methanol: H3+ e- CH2DOH CH2DOHH+ CH2DOH Thus, CH3OD will be converted to CH3OH in the gas-phase after evaporation. H3+ e- CH3OD CH3OD CH3ODH+ e- CH3OH The three methanol species which were observed to have high fractionation in IRAS16293 are those with the deuterium in the methyl group: CH2DOH, CHD2OH, CD3OH. • So we make a simple protostellar core model: • input abundances from the end-point of the prestellar core phase • PLUS H2O, H2CO,CH3OH and H2S from `surface chemistry’. • We obtain the molecular D/H ratios for these molecules from the model of Stantcheva et al. (2003) assuming an accreting D/H ratio of 0.3.

  16. The initial fractional abundance and D/H ratios assumed for methanol • As expected, the abundances of those isotopomers with an –OD group decline faster. If we are confident about the model parameters (!), then the observed relative abundances of these species could tell us the age of the protostar (a `chemical clock’). OR… If we know the age of the protostar, then these observations could constrain other parameters (e.g. the cosmic ray ionisation rate). Results from a `protostellar’ model, assuming the grain mantles evaporate at t = 0yr. T= 50K; n(H2) = 106cm-3 (Osamura et al. 2004).

  17. Conclusions: Deuterium bearing molecules are extremely useful probes of conditions in interstellar and protostellar regions. The chemical models rely on theoretical determinations and laboratory measurements of rate coefficients both in the gas-phase and on grain surfaces. Dark Clouds Do deuterated species react with the same rates as their analogues? Prestellar Cores H2D+ and HD2+ observations may prove useful in determining the ionisation fraction. Are accretion models sufficient, or is desorption from grains important even at low temperatures? Protostellar Cores Where species have evaporated from grain-surfaces and subsequently reacted, observations give us clues about the surface chemistry, but more theoretical and experimental data is required for a coupled gas-grain model.

More Related