AstrochemistryLecture 9, Chemistry in cold and hot cores

Jorma Harju

Department of Physics

Friday, March 22, 2013, 12:15-13:45, Lecture room D117Course web page

http://www.courses.physics.helsinki.fi/astro/astrokemia

Dark cloud

I Cold (T ∼ 10− 20K ), relatively dense (n > 104 cm−3) molecularcloud

IOn a photographic plate, (al-most) starless spot owing to ab-sorption and scattering by dust

I Observable through thermal dust emission (λ ∼ 0.2− 1mm) andmolecular line emission (e.g. CO rotational lines)

I Chemistry is dominated by ion-molecule reactions, radicalreactions, formation of carbon chains

Dense core

I Large scale turbulence gives rise to condensations (n > 105

cm−3) that can contract and finally collapse under gravity

I Thermal pressure and small scale turbulence sustained by themagnetic fields can retard or prevent gravitational collapse

I In molecular clouds there are both starless and star-formingcores

I Starless cores are heated by the interstellar radiation field andcosmic rays-density increases but the temperature decreases inwards

Distribution of molecules (1)

Dense core L1498 in Tau-rus in the lines of CS,CCS, and NH3 (Willacy etal. 1998)The distributions of NH3and CCS are completelydifferent

Distribution of molecules (2)

L1544 (in Taurus): thermal dustemission (contours) and N2H+

line emission maps are simi-lar, C17O avoids in the nucleus(Caselli et al. 1999)

Distribution of molecules (3)

L1544: maps in dustcontinuum and severalmolecular lines (Tafalla etal. 2006)

Freezing of molecules (1)

I The observed distributions can be understood in terms ofadsorption of neutral atoms and molecules on grain surfaces

I The formation of icy mantles is accompanied by grain-surfacereactions, converting O, C, N, and CO (when they react with H orO) to H2O, CH4, NH3, H2CO, CH3OH, and CO2

I Molecular abundances in the gas phase depend on the relativeefficiencies of adsorption and desorption mechanismsThis depends on the gas density and temperature, species,cosmic ray bombardment, and radiation field

Freezing of molecules (2)

I Nitrogen-containing molecules like N2H+ and NH3 seem to beable to resist depletion at higher densities than carboncompounds, e.g. CO and CS

I Reasons for relatively high abundances of N-containing speciesin dense cores (see Lecture 2):- N2 photodissociates easily- N2 formation begins with slow neutral-neutral reactions- The production of N2H+ and NH3 may be temporarily favouredby an increase in the H+

3 abundance

Effects of depletion (1)

I The abundance of the H+3 ion increases when neutral molecules

and atoms disappear because reactions likeH+

3 + CO→ HCO+ + H2

H+3 + N2 → N2H+ + H2

occur less frequently

I The reduced destruction of H+3 results in an enhanced deuterium

fractionation:H+

3 + HD↔ H2D+ + H2 + ∆EThe reaction proceeding right is exothermic, ∆E/k = 232 K

I H2D+ transfers deuterium to other molecules:

H2D+ + CO → DCO+ + H2HCO+ + HD

H2D+ + N2 → N2D+ + H2H2H+ + HD

Effects of depletion (2)

I The deuteration sequence continuesH+

3 → H2D+ → D2H+ → D+3 :

H2D+ + HD → D2H+ + H2D2H+ + HD → D+

3 + H2

Other products are possible, but the forward reactions listedabove are exothermic by 187 K and 234 K, respectively

I Unexpectedly strong emission lines of H2D+ and D2H+ detectedin cold cores (Caselli et al. 2003, A&A 403, L37; Vastel et al.2006, ApJ 645, 1198)

I The presence of (singly and multiply) deuterated forms of H+3 is

manifest in relative large abundances of deuterated molecules,e.g. N2D+, NH2D, ND2H, ND3

Effects of depletion (3)

I It has been suggested that when the density exceeds ∼ 106

cm−3, only hydrogen, deuterium, and helium are left in the gasphase (“complete depletion”, Walmsley, Flower & Pineau desForêts, 2004, A&A 418, 1035)

I The ortho:para ratio of H2 affects deuteration: the leftwardreactionH+

3 + HD↔ H2D+ + H2

is viable also in cold clouds with ortho-H2 (170 K abovepara-H2), and is exothermic foroH2D+ + oH2 → pH+

3 + HD

I ortho-H2 acts as a storage of chemical energy, and can preventdeuteration

Effects of depletion (4)

I The orto:para ratio of H2 is 3 : 1 at formation, but decreases inthe gas phase, predominantly through the reactionsH+ + oH2 → H+ + pH2,p/oH+

3 + oH2 → o/pH+3 + pH2

I Models predict the o/p-H2 is thermalized down to T ∼ 20 K(o/p ∼ 10−3) but drags behind when the temperature decreasesduring the cloud contraction, and never reaches thermalequilibrium at ∼ 10 K

I Recent results by Olli Sipilä suggest, furthermore, thatdeuteration at lates stages can be limited by HD depletion owingto grain-surface reactions (D incorporated in ices)

Molecular probes of cold cores

I At high densities, nH2 > 106 cm−3, H+3 and its deuterated forms

increase strongly

I Asymmetric species H2D+ and D2H+ (with permanent electricdipole moment) are important tools - the emission comesexclusively from dense parts

I The lowest rotational transitions of ortho-H2D+ and para-D2H+

(372 and 692 GHz) observable from the ground(only in excellent atmospheric conditions: Chajnantor (APEX,ALMA) and Mauna Kea)

I The ground-state transitions of para-H2D+ and ortho-D2H+

(1370 and 1477 GHz) need space

Rotational energy levels of H2D+ and D2H+

(Pagani et al. 1992, Vastel et al. 2004)

ρ Oph (1)

The ρ Oph molecular cloud complex in far-infrared and inCO(J = 1− 0) line emission (Kirk et al. 2005)

ρ Oph (2)

ρ Oph in mid-infrared (Spitzer)

H2D+ lines from ρ Oph cores

H2D+ spectrum from the core Oph D

ISOCAM 7µm

8000 AU

H2D+ line in the starlees core Oph D (APEX)

I Line width ∆v(FWHM) = 0.26± 0.03 kms−1 impliesTkin = 6.0± 1.4 K (maximum assuming thermal broadening andoptically thin emission)

I Direct evidence for a very low temperature in the nuclei ofstarless cores

Other observations of Oph D

N2D+ map (Crapsi et al 2005): H2D+ signal comes from the N2D+

peak

H2D+ ja D2H+ in ρ Oph H-MM1

Parise et al. (2011 A&A 526, A31), APEX/CHAMP+ array

Star formation (1)

I During star formation, thegas experiences largechanges in the density andtemperature

I The core collapse,protostellar outflows, andstellar radiation give rise topowerful shocks

I A protostellar envelopecan have large gradients inn and T ,n ∼ 109 − 104 cm−3,T ∼ 300− 10 K,at radii r ∼ 1015 − 1017 cm

Hot cores (1)

I The effects of star formation are particularly dramatic in hotcores surrounding massive young stars

I Hot cores are compact (< 0.1 pc), warm (T > 100 K) and dense(n > 106 cm−3) objectsTheir spectra are characterized with forests of lines from organicmolecules

Hot cores (2)

I The density and temperature profiles in a protostellar envelopeare often described by power-laws, for example,n(r) ∝ rα, α ∼ −1.0→ −1.5T (r) ∝ rβ , β ∼ −0.5in the range rin ≤ r ≤ rout - both density and temperatureincrease inwards

I The temperature rise is predominantly caused by the protostellarUV and X-ray radiation (accretion luminosity) which heats thegas and dust

I In the outer parts, conditions can resemble those in cold cores,T ∼ 10 K.

Hot cores (3)

I In the collapse phase, molecules and atoms are incorporated inicy mantles around dust grainsAbsorption lines from ices seen in infrared spectra towards hotembedded sources - these originate in the cool outer parts

I In the inner regions ices evaporate: emission lines from gaseousH2O, CH3OH, CH4, ...

I In bipolar outflows with T ∼ 200− 2000 K, n ∼ 105 − 107 cm−3

shock heating give rise to high-T chemistry with H2O, OH, O2(Goldsmith et al. 2011)Disruption of dust grains: SiO, SiO2

Evaporation in hot cores (1)

I Pure CO ice evaporates at about 20 KEvaporation temperatures in lab (K, according to Mumma et al. PPIII, 1993)

N2 CO CH4 H2S H2CO CO2 NH3 SO2 HCN CH3OH H2O22 25 31 57 64 72 78 83 95 99 150

Evaporation temperatures are lower in space because of the lowpressure

I All ices are evaporated when temperature has risen to∼ 100− 150 KMolecules like methanol (CH3OH), water (H2O) and hydrogensulfide (H2S) are released into the gas phase

Evaporation in hot cores (2)

I Sublimates like HCO, CH3, OH, OCS, H2S, ... react in the warmgas forming organic compounds

I Probably also association reactions in the ices are important

I Besides the “common” species like CO, SO, SO2, HCN, NH3,...also the following molecules are abundant: H2CO, CH3OH,HCOOH, HCOOCH3, CH3OCH3, CH2CHCN

I Close to the central star (T ∼ 200− 300 K) neutral gas-phasereactions produce plenty of water which binds most of oxygen

Sublimation zones

picture: Ewine van Dishoeck

Hot core spectrum

picture: Ewine van Dishoeck

Orion KL (1)Herschel HIFI spectrum around 1.1 THz

Orion KL (2)Distributions of selected molecules, Plateau de Bureinterferometer

Surface reactions (1)

I In the ice mantles saturated molecules are formed when Hassociates with O or N: H2O, CH4, NH3, CH3OH, H2S

I Saturated species can dissociate into radicals under theinfluence of radiation (e.g. secundary photons produced incosmic ray collisions with the gas)

I CO formed in the gas phase freezes directly onto dust grainsHydrogenation produces formaldehyde H2CO, but also theradicals HCO and CH3O:H(s) + CO(s) → HCO(s)

H(s) + H2CO(s) → CH3O(s)

I CO2 and CO are equally abundant in interstellar icesCO/CO2 ice probably dominates the outer layers of ice mantles,whereaas the inner layers are dominates by H2O

Surface reactions (2)

I The heating by an embedded protostar affects the chemistryboth in the gas phase and on grain surfaces

I H atoms and CO escape quickly from the ice mantles when thetemperature rises to ∼ 20 KFormaldehyde evaporates at T ∼ 40 K.

I Desorption and also the diffusion of molecules in the ice dependexponentially on T accroding to the Boltzmann distributionDesorption energies , ED (K), according to Garrod & Herbst(2006, A&A 457, 927)

H 450 H2 430 OH 2850 H2O 5700 N2 1000CO 1150 CH4 1300 HCO 1600 H2CO 2050 CH3O 5080CH3OH 5530 HCOOH 5570 HCOOCH3 6300 CH3OCH3 3150

I The diffusion barrier is commonly assumed to be half of thedesorption energy

Surface reactions (3)

I Complex species can be formed both in the gas phase or in theices when the mobility of molecules is increased owing towarming up

I For example, formic acid HCOOH:a) HCO+(g) + H2O(g) → HCOOH+

2 (g) + hν ,HCOOH+

2 (g) + e− → HCOOH + H

b) OH(g) + H2CO(g) → HCOOH(g) + H(g)

c) HCO(s) + OH(s) → HCOOH(s))

I For large molecules, electron recombination results easily infragmentationTherefore surface reactions are probably important

I For example, dimethyl ether is supposed to have beenevaporated from ices after the reactionCH3(s) + CH3O(s) → CH3OCH3(s)

Methyl formate

I Methyl formate (the methyl ester of formic acid), HCOOCH3, iscommon in hot coresProduced both in ices and in the gas, e.g.;HCO(s) + CH3O(s)→ HCOOCH3(s)

I Two isomers: CH3COOH (acetic acid), CH2OCHO (glycolaldehyde)Both detected but more rare than HCOOCH3.

methyl formate acetic acid glycol aldehyde

Acetic acid

I Suggested reactions:CO(s) + OH(s)→ COOH(s) ,CH3(s) + COOH(s)→ CH3COOH(s)

CH3OH+2 (g) + HCOOH→ CH3COOH+

2 (g) + H2O(g) ,CH3COOH+

2 (g) + e− → CH3COOH(g) + H

I The structure of acetic acid is similar to that of the simplestamino acid glycine, NH2CH2COOHPossible formation reaction in the ice:NH2(s) + CH3COOH(s)→ NH2CH2COOH(s)

I Glycine has not been detected with certainty in the interstellarspace

Sulfur chemistry in hot cores (1)

I The chemistry of sulfur begin with the evaporation of H2S fromices

I H2S is believed to form in the ice via hydration of S

I In the gas phase H2S requires a high temperature, and it ispossibly formed in shocks

I Sulfur can be released from hydrogen sulfide in reactions with Hatoms or the OH radical, for example

H2S H−→HS H−→ S

Sulfur chemistry in hot cores (2)

I Sulfur monoxide: SH + O→ SO + Hor S + OH→ SO + Hor S + O2 → SO + O

I Sulfur dioxide SO + OH→ SO2 + H (if OH available)

I Most sulfur is converted into SO2 in hot cores

I On grounds of the reaction chain H2S→ SO→ SO2 theabundance ratios [SO]/[H2S] and [SO2]/[SO] are sometimesused in attempts to estimate the age of the object

I Other simple sulfur species in hot cores: CS (carbonmonosulfide), OCS (carbonyl sulfide), H2CS(“thioformaldehyde”), S2 (sulfur dimer)

Evolution of hot cores (1)

Garrod & Herbst (2006, A&A 457, 927) - collapse phase

Chemistry during the collapse phase. Surface species are designated by (s).

Evolution of hot cores (2)

Garrod & Herbst (2006) - warm-up phase