The Ionization Toward The High-Mass Star-Forming Region NGC 6334 I Jorge L. Morales Ortiz 1,2 (Ph.D. Student) C. Ceccarelli 2, D. Lis 3, L. Olmi 1,4, R. Plume 5, and P. Schilke 6 1 University of Puerto Rico - Río Piedras, 2 Insitute de Planétologie et d’Astrophysique de Grenoble, 3 California Institute of Technology, 4 Osservatorio Astrofisico di Arcetri, 5 University of Calgary, 6 I. Physikalisches Institut der Universität zu Köln

Outline Introduction and observations Results from radiative transfer analysis Results from chemical modeling Summary

The NGC 6334 I Hot Molecular Core Optical image (NOAO) I NGC 6334 High-mass star-forming region in the Galactic Plane Distance ~ 1.7kpc Mass ~ 200 M , T dust ≥ 100 K, L ~ 2.6 x 10 5 L  Separated from other SF sites in NGC6334  Can be studied in an isolated manner and at smaller spatial scales

Spectral Line Observations: NGC 6334 I Herschel/HIFI observations of high-energy rotational transitions (J up ≥ 5) of the molecular ions HCO + and N 2 H +, and isotopologues H 13 CO +, C 18 O, and C 17 O 1.3mm dust continuum emission from SMA observations (Hunter et al. 2006) NGC 6334 I core consists of 4 compact condensations within a 10” diameter region Spatial structure is unresolved by our Herschel/HIFI observations  line profile depends on all emission components Zernickel et al. (2012)

Results: Molecular Line Spectra Detection of several rotational transitions: – HCO +  7 lines – N 2 H +  4 lines – C 18 O  7 lines – C 17 O  5lines – H 13 CO +  3 lines Molecular lines with 79 K ≤ E up ≤ 348 K  Ability to trace different physical components within the region

Results: Spectral Line Profiles HCO + and C 18 O suffer from optical depth effects HCO + / H 13 CO + = 18 (< 12 C / 13 C ≈ 75) C 18 O / C 17 O = 2.8 (< 18 O / 17 O ≈ 3.5) Red-shifted asymmetry in line profiles of HCO +, N 2 H +, C 18 O, and C 17 O

Results: Spectral Line Profiles HCO + and C 18 O suffer from optical depth effects HCO + / H 13 CO + = 18 (< 12 C / 13 C ≈ 75) C 18 O / C 17 O = 2.8 (< 18 O / 17 O ≈ 3.5) Red-shifted asymmetry in line profiles of HCO +, N 2 H +, C 18 O, and C 17 O  Profile consistent with expanding gas  Gaussian fits suggest a velocity gradient in the emitting gas

Results: Radiative Transfer Analysis Both molecules fitted simultaneously Two-component model needed for a proper fit C 18 O & C 17 OHCO + & H 13 CO + non-LTE radiative transfer code with Large Velocity Gradient (LVG) approximation (Ceccarelli et al. 2003) Model the spectral line emission to estimate physical parameters (T, n, θ, N)

Results: Radiative Transfer Analysis Both molecules fitted simultaneously Two-component model needed for a proper fit  Spectra are composed of two physical components corresponding to two regions of the envelope T = 60 K n H2 = 10 6 cm -3 θ = 9 arcsec T = 35 K n H2 = 10 5 cm -3 θ = 40 arcsec C 18 O & C 17 O HCO + & H 13 CO +

Results: Radial Structure of NGC 6334 I  Results from LVG analysis are remarkably consistent with the radial structure of the envelope Radiative transfer model by Rolffs et al. (2011) Model assumes a centrally heated sphere with a power- law density gradient Radial profiles of density and temperature from dust continuum emission at 850μm from APEX telescope Density (squares) and temperature (triangles) values obtained from our LVG analysis are overlaid

Results: Origin of the Emission

 Detection of the expanding envelope surrounding the hot core of NGC 6334 I  The outer envelope is expanding with respect to the inner envelope with a velocity of 1.5 km/s. Thermal pressure from hot ionized gas, P t / k = 1.6 x10 9 cm -3 K Ambient pressure exerted by the envelope, P a / k = 10 8 – 10 9 cm -3 K  Thermal pressure drives the envelope expansion

Results: Column Densities and Molecular Abundances The molecular column densities from the LVG analysis are used to estimate the relative abundances between the various molecular species  The HCO + and CO abundances in the inner envelope are higher when compared to the abundances in the outer envelope, while the opposite is true for the N 2 H + abundance.  For [CO] / [H 2 ] abundances of ~ 10 -4, N 2 H + is destroyed through reactions with CO, forming HCO + in the process

Results: Chemical Modeling Given that the production of HCO + and N 2 H + in dense regions is dominated by the cosmic ray ionization rate (ζ), we can give an estimate of ζ from their molecular abundances Nahoon code from Wakelam et al. (2012) – Compute the chemical evolution at a fixed T, n, and ζ using the gas-phase reaction network from KIDA (Kinetic Database for Astrochemistry;

Results: Cosmic Ray Ionization Rates ComponentRadiative transfer modelNahoon / KIDA model n H2 [cm -3 ][HCO + ] / [N 2 H + ]ζ [s -1 ][HCO + ] / [N 2 H + ] Outer Envelope Inner Envelope T = 35 K T = 60 K [HCO + ]/[N 2 H + ] H H Grid of density and cosmic ray (CR) ionization rates  Estimate ζ for NGC 6334 I given the derived physical parameters from LVG analysis

Results: Cosmic Ray Ionization Rates ComponentRadiative transfer modelNahoon / KIDA model n H2 [cm -3 ][HCO + ] / [N 2 H + ]ζ [s -1 ][HCO + ] / [N 2 H + ] Outer Envelope x Inner Envelope x T = 35 K T = 60 K [HCO + ]/[N 2 H + ] H H Grid of density and cosmic ray (CR) ionization rates  Estimate ζ for NGC 6334 I given the derived physical parameters from LVG analysis Fix density Estimate ζ

Results: Cosmic Ray Ionization Rates The CR ionization rate in the outer envelope is approximately two times higher than in the inner envelope of NGC 6334 I  Main source of ionization originates outside of NGC 6334 I The X-ray ionization from NGC 6334 I core (Sekimoto et al. 2000) is comparable to the CR ionization up to a radius of 0.05 pc for the outer envelope (r = 0.16 pc) and up to a radius of 0.07 pc for the inner envelope (r = 0.04 pc)  Ionization in the outer envelope is dominated by CRs  Ionization in the inner envelope has an additional contribution from the X-ray emission  upper limit for CR ionization rate

Summary The analysis of NGC 6334 I has allowed us to give a better description of the gas kinematics in the envelope of this hot molecular core We identify two physical components in the molecular line spectra, each with a different excitation, corresponding to emission from the envelope From the observations, we conclude that there is an expansion of the envelope surrounding the hot core of NGC 6334 I, which is driven by thermal pressure from the hot ionized gas in the region The ionization rate is dominated by CRs originating from outside the source, although X-ray emission from the core could contribute to the ionization in the inner region of the envelope

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