OBSERVATIONS OF INTERSTELLAR HYDROGEN FLUORIDE AND

HYDROGEN CHLORIDE IN THE GALAXY

Raquel R. Monje Darek C. Lis, Thomas Phillips, Paul

F. Goldsmith Martin Emprechtinger, David Neufeld and

Maryvonne Gerin

• Fluorine, with IP > IP(H), D0(HX) > D0(H2). Fluorine is predominantly neutral in the diffuse ISM, and can react exothermically with H2 and HF.

• Chlorine, with IP < IP(H), D0(HX) < D0(H2) and D0(HX+) > D0(H2). Chlorine is predominantly ionized in atomic clouds. Cl+ can react exothermically with H2 to form HCl+

The Unique Thermochemistry of Fluorine and Chlorine

• Once ionized Cl+ reacts with molecular hydrogen exothermically to form HCl+, a process that initiates the chemical reactions of chlorine.

• In cloud interiors, atomic chlorine is mostly neutral, not ionized, so that the reaction of Cl and H3

+ drives the formation of H2Cl+, which then reacts with CO and H2O leading to the formation of HCl.

• An alternative direct route to HCl in either hot gas or regions where vibrationally excited H2 is abundant is the Cl+H2->HCl+H reaction.

• The destruction of HCl is dominated both by photoionization and photodissociation and by reactions with C+ and H3

+.

The HCl Chemistry

Source D(kpc)

W49N 11.5W31C 4.8

SgrA(+50 km/s cloud) 8.6W28A 3.8

G34.3+0.1 3.8W51 7

W33A 4DR21(OH) 1.0

• Q= 10”, Tex =40K,

• Ntot= 3.0 x 1014 cm-2,

• DV=4 km/s, VLSR=-4km/s• [H35Cl/H37Cl] ratio of 2.7 (3.1 the solar isotopic abundance)

Results from an initial LTE model:

HCl toward DR21(OH)

Intensity ration of 2.7:3:2(Optically thin 1:2:3)(Optically thick 1:1:1)

HCl toward W31C self absorption feature at VLSR =-1 km/s

Absorption by foreground in the VLSR range 15-50 km/s

Neufeld et al, 2010, 518,L108

HCl toward W31C: The ForegroundHFS fit + subtraction of absorption

Decomposition of the Hyperfine components

Courtesy of B. Godard’s Code

VLSR N(HCl) HCl/H2a

20 - 25 1.5 x 1013 5.42 x 10-9

25 – 35 2.6 x 1013 3.52 x 10-9

35 - 45 3.1 x 1013 5.64 x 10-9

aH2 column densities derived from CH observations, Gerin et al. 2010, assuming N(CH) = 3.5x10-8 N(H2).

HCl toward W49N

Self absorption from the envelope Outflows?

Emission from other lines?

HCl toward SgrA+50 km/s cloudHyperfine decomposition

Ntot= 7.17 x 1014 cm-2

[H35Cl/H37Cl] ratio of 2.66

HCl toward W33A

• Fluorine is the only atom that can react exothermically with H2 to form a diatomic hydride:

• HF is destroyed very slowly by means of cosmic ray-induced photodissociation and as a result of reactions with species of low abundances such as He+,H3

+, and C+.

• HF is expected to be the dominant reservoir of gas-phase fluorine. N(HF)/N(H2) ~ 3.6 x 10-8

• HF may be used as a valuable surrogate tracer for molecular hydrogen within diffuse interstellar medium, both in the Milky Way and other galaxies.

F + H2 → HF + H +1.4eV

HF in the ISM

NGC 6634 I

W51

W49

W31C

Orion KL

Sgr B2(M)

Sgr A+50

NGC 6334I

Sgr B2(N)

Sgr A+50 km/s

Sgr B2(M)

Toward the Galactic center

Monje et al. 2011, Apj, 734, L23Sonnentrucker et al. in preparation

Object VLSR

km s-1N(HF)

x 1013 cm-2HF/H2

x 10-8

Sgr B2(M)a -115 – -60-53 – -49-35 – -28-10 – -526 – 40

9.91.11.3

1.011.7

1.71.51.41.11.1

W49N b 30 – 5050 – 7867 – 71

55695.6

1.51.11.3

W51b 0 – 1010 – 2020 – 30 42 – 47

14.51.80.88.0

1.41.3

>1.21.6

G34b 3 – 1325 – 3545 – 50

1.5 1.22.2

1.61.32.5

Estimates of the HF/H2 ratio

-200 -150 -100 -50 0 50 1000

0.5

1

1.5

2

2.5

3

Sgr B2(M)W49NW51G34

vLSR (km s-1)

N(H

F)/N

(H2)

x 1

0-8

aH2 column densities derived from 13CO assuming CO abundances of 3x10-5N(H2) (Sonnentucker 2005).bH2 column densities derived from CH observations, Gerin et al. 2010, assuming N(CH) = 3.5x10 -8 N(H2).

Conclusions• Observations of H35Cl and H37Cl, and accurate measurements of the

35Cl/37Cl ratio in different environments, will provide some insight into the chemical evolution of both isotopes, thus into Galactic chemical evolution.

• Firs detection of HCl in diffuse medium toward the line-of-sight of W31C.

• One key result from Herschel/HIFI is the ubiquitous nature of hydrogen fluoride within the interstellar medium of the Milky Way.

• The HF abundances of 1-2 x 10-8 under a wide variety of interstellar conditions are consistent with the chemical models.

• HF shows good promise as a tracer of H2.

• HCl is the most abundant Cl-bearing molecule in dense clouds.

• Both 35CL and 37CL nuclei are believed to form in the last burning stages of massive stars (>10Msun) and by means of “explosive nucleosynthesis” during supernovae detonation.

• Observations of H35Cl and H37Cl, and accurate measurements of the 35Cl/37Cl ratio in different environments, can provide some insight into the chemical evolution of both isotopes, thus into Galactic chemical evolution

HCl in the ISM

Neufeld et al. 1997, 488, L141

HF in the ISM- A brief history -

• First detection of interstellar HF J 2-1.• Towards Sgr B2 using LWS on ISO. • HF abundances of ~ 3 x 10-10.

ISO

Extraordinary Source: Orion KL

HF/H lower limit ~ 1.6 x 10-10

Phillips et al. 2010, A&A, 518, 109

Extragalactic HF

van der Werf et al. 2010, A&A, 518, L42

Mrk 231 with SPIRE

HF J = 1-0 detection with the CSO with after ~10hr

Monje, Phillips, Peng et al. in preparation

Cloverleaf: QSO at z = 2.56