Astrochemistry: From primordial gas to present-day clouds
Dominik R.G. Schleicher, Stefano Bovino, Bastian Körtgen, Tommaso Grassi, Robi Banerjee
aa r X i v : . [ a s t r o - ph . GA ] A ug Mem. S.A.It. Vol. , 1 c (cid:13) SAIt 2008
Memorie della
Astrochemistry: From primordial gas topresent-day clouds
Dominik R.G. Schleicher , Stefano Bovino , Bastian K¨ortgen , TommasoGrassi , and Robi Banerjee Departamento de Astronom´ıa, Universidad de Concepci´on, Barrio Universitario,Concepci´on, Chile e-mail: [email protected] Hamburger Sternwarte, Universit¨at Hamburg, Gojenbergsweg 112, 21029Hamburg, Germany Centre for Star and Planet Formation, Niels Bohr Institute & Natural HistoryMuseum of Denmark, University of Copenhagen, Oster Voldgade 5-7, DK-1350Copenhagen, Denmark
Abstract.
Astrochemistry plays a central role during the process of star forma-tion, both in the primordial regime as well as in the present-day Universe. Werevisit here the chemistry in both regimes, focusing first on the chemistry un-der close to primordial conditions, as observed in the so-called Caffau star SDSSJ102915+172927, and subsequently discuss deuteration processes in present-daystar-forming cores. In models of the high-redshift Universe, the chemistry is par-ticularly relevant to determine the cooling, while it also serves as an importantdiagnostic in the case of present-day star formation.
Key words.
Astrochemistry – Stars: formation – primordial Universe – molecularclouds
1. Introduction
Chemistry plays a central role in astro-physics, and particularly during star for-mation. In the primordial Universe consist-ing only of hydrogen and helium, the H molecule provides the only and relativelyinefficient coolant, implying relatively hightemperatures of the star-forming cloudscompared to Milky Way type conditions. Inthe absence of dust grains, H abundancesof order 10 − start forming at gas numberdensities of 10 cm − due to gas phase re-actions (Saslaw & Zipoy, 1967). The gasbecomes fully molecular from densities of about 10 cm − due to three-body reac-tions (Palla et al., 1983). The resulting tem-peratures of 300 K or higher are commonlyexpected to give rise to significantly en-hanced masses of the first stars.The presence of even tiny amounts ofdust grains may alter this picture, givingrise to potentially strong fragmentation athigh densities (Schneider et al., 2003). Thestar SDSS J102915+172927 is consideredas a candidate for such behavior (Klessenet al., 2012; Schneider et al., 2012), asits metal abundances are so low that onlydust grains could have contributed rele- Schleicher, Bovino, K¨ortgen & Grassi: Astrochemistry and star formation vantly to the cooling (Caffau et al., 2011).In the following, we will present a numer-ical simulation exploring how the forma-tion of such a star may have occurred. Wewill subsequently turn to deuteration pro-cesses in present-day clouds, as explored byWalmsley et al. (2004). We will show thatlarge deuteration fractions can be reachedwithin about a free-fall time. The chemicalmodeling pursued in this work is based onthe publicly available astrochemistry pack-age KROME (Grassi et al., 2014).
2. The formation of the Caffau starSDSS J102915+172927
We model the formation of the Caffau starusing the cosmological hydrodynamics codeEnzo (Bryan et al., 2014) combined withthe astrochemistry package KROME. Weexplore the results for two minihalos, onewith 10 M ⊙ and one with 7 × M ⊙ ,forming at z = 22 and z = 18, respec-tively. The halos are part of a cosmologi-cal box with size of 300 kpc h − , an initialtopgrid resolution of 128 , two additionalnested grids around the halo of choice and29 levels of refinement. The Jeans length isalways resolved with at least 64 cells. Thedetails of the setup, as well as our treat-ment of the chemistry and the dust grains,is reported by Bovino et al. (2016). Fixingthe metal abundances to the values of theCaffau star, we find that the cooling is pre-dominantly regulated by the depletion fac-tor f dep = D/Z , where D denotes the dust-to-gas mass ratio and Z the metallicity.The resulting thermal evolution andthe density structure is shown in Fig. 1,where higher depletion factors correspondto lower gas temperatures at high densi-ties. Looking also at the density structure,we found that a strong transition from anapproximately spherical collapse mode to a filamentary collapse occurs between de-pletion factors of 0 . .
49, suggest-ing that the latter corresponds to the crit-ical threshold for fragmentation inducedvia dust cooling for metallicities as in theCaffau star (Bovino et al., 2016).
3. Deuteration processes duringpresent-day star formation
We further explore deuteration processesin prestellar cores, with the aim of infer-ring the approximate timescale to reachhigh deuteration fraction. For this purpose,we consider the collapse of a turbulentmagnetized Bonnor-Ebert sphere (Ebert,1955; Bonnor, 1956), which is modeled withthe magneto-hydrodynamical code FLASH(Fryxell et al., 2000). The chemistry ismodeled using the network of Walmsleyet al. (2004) under the assumption of fulldepletion (see K¨ortgen et al., 2017, for adetailed description of the overall setup).In Fig. 2, we show the radial profiles ofthe deuteration fraction and the spin statesof H D + for runs with different initial H ortho-to-para ratios, exploring values of 3,1 and 0.1. Results are given at 15 kyrs,42 kyrs, 63 kyrs and 75 kyrs, correspond-ing to 0 .
1, 0 .
28, 0 .
42 and 0 . ortho-to-para ratio of 3, a deuter-ation fraction of 0.01 is achieved withinthe central 1000 AU during half a free-falltime. This demonstrates the high efficiencyof deuteration processes, which we will ex-plore in more detail in future investigationsincluding finite amounts of depletion. Acknowledgements.
We thank Francesco andMalcolm for the time they spent with us.
References
Bonnor, W. B. 1956, MNRAS, 116, 351Bovino, S., Grassi, T., Schleicher, D. R. G.,& Banerjee, R. 2016, ApJ, 832, 154Bryan, G. L., Norman, M. L., O’Shea,B. W., et al. 2014, ApJS, 211, 19Caffau, E., Bonifacio, P., Fran¸cois, P., et al.2011, Nature, 477, 67 chleicher, Bovino, K¨ortgen & Grassi: Astrochemistry and star formation 3 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 Density[g/cm ] T e m p e r a t u r e [ K ] run1: PRIMORDIALrun2: fdep=0.0082run3: fdep=0.015run4: fdep=0.03run5: fdep=0.10run6: fdep=0.49run7: α = -3.5, fdep=0.49 Fig. 1.
Profile of the mass-weighted average temperatures for different dust depletionfactors f dep in the dark matter halo with 10 M ⊙ (left), as well as the density structureon a scale of 20 AU for selected runs with both halos (left: 10 M ⊙ , right: 7 × M ⊙ )(Bovino et al., 2016). The calculations assume the metal abundances of the Caffau star. ✲ (cid:0)✲ ✁✲ ✂✲ ✄✵ ✄ ✵ ✵ ✵ ✄ ✵ ✵ ✵ ✵ ❧ ☎✆ ✝ ✞ ❢ ✟ ✠ ✡ ❍ ✷ ❉ ✰ ✮ ❘ ☛ ☞ ✌ ✍ ✎ ✏ ✑ ✒ ✓t ✔ ✕ ✖ ✗ ✘ ✙ ✔ ✚ ✛ ✕ ✚ t ✜ ✜t ✔ ✢ ✣ ✗ ✘ ✙ ✔ ✚ ✛ ✣ ✤ t ✜ ✜t ✔ ✥ ✦ ✗ ✘ ✙ ✔ ✚ ✛ ✢ ✣ t ✜ ✜t ✔ ✧ ✖ ✗ ✘ ✙ ✔ ✚ ✛ ✖ ✚ t ✜ ✜ ▲ ★ ✩ ✪ ✫ ✬ ✭ ✯ ✱ ✳ ✴▲ ★ ✩ ✪ ✫ ✬ ✭ ✯ ✱ ✳ ✫ ✶ ✪▲ ★ ✩ ✪ ✫ ✬ ✭ ✯ ✱ ✳ ✪ ✾✶ (cid:0)✶ ✶✶ ✁✶ ✂✶ ✄✶ ☎ ✶ (cid:0) (cid:0) (cid:0) ✶ (cid:0) (cid:0) (cid:0) (cid:0) ❧ ✆✝ ✞ ✆ ✟ ✠ ✷ ❉ ✰ ✥ ✡ ☛ ✲ ✷ ❪ ☞ ❘ ✌ ✍ ✎ ✏ ✑ ✒ ✓ ✔ ✕▲ ✖ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✤▲ ✖ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✙ ✦ ✘▲ ✖ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✘ ✾✶ (cid:0)✶ ✶✶ ✁✶ ✂✶ ✄✶ ☎ ✶ (cid:0) (cid:0) (cid:0) ✶ (cid:0) (cid:0) (cid:0) (cid:0) ❧ ✆✝ ✞ ✟ ✠ ✡ ✷ ❉ ✰ ✥ ☛ ☞ ✲ ✷ ❪ ✌ ❘ ✍ ✎ ✏ ✑ ✒ ✓ ✔ ✕ ✖▲ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✤ ✦▲ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✤ ✚ ✧ ✙▲ ✗ ✘ ✙ ✚ ✛ ✜ ✢ ✣ ✤ ✙ Fig. 2.
Comparison of radial profiles of the deuteration fraction and the spin states ofH D + for runs with initial H2