Transcript Jorge_Morales-OSU_MSS_June2013x

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
• High-mass star-forming region in
the Galactic Plane
NGC 6334
• Distance ~ 1.7kpc
• Mass ~ 200 M, Tdust ≥ 100 K, L ~
I
2.6 x 105 L
• Separated from other SF sites in
NGC6334
 Can be studied in an isolated
manner and at smaller spatial
scales
Optical image (NOAO)
Spectral Line Observations: NGC 6334 I
• Herschel/HIFI observations of high-energy rotational transitions (Jup ≥ 5) of the
molecular ions HCO+ and N2H+, and isotopologues H13CO+, C18O, and C17O
•
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
0.1 pc
Zernickel et al. (2012)
Results: Molecular Line Spectra
• Detection of several rotational transitions:
– HCO+  7 lines
– N2H+  4 lines
– C18O  7 lines
– C17O  5lines
– H13CO+  3 lines
• Molecular lines with 79 K ≤ Eup ≤ 348 K
 Ability to trace different physical
components within the region
Results: Spectral Line Profiles
•
HCO+ and C18O suffer from optical
depth effects
•
HCO+ / H13CO+ = 18 (< 12C / 13C ≈ 75)
•
C18O / C17O = 2.8 (< 18O / 17O ≈ 3.5)
• Red-shifted asymmetry in line
profiles of HCO+, N2H+, C18O, and
C17O
Results: Spectral Line Profiles
•
HCO+ and C18O suffer from optical
depth effects
•
HCO+ / H13CO+ = 18 (< 12C / 13C ≈ 75)
•
C18O / C17O = 2.8 (< 18O / 17O ≈ 3.5)
• Red-shifted asymmetry in line
profiles of HCO+, N2H+, C18O, and
C17O
 Profile consistent with expanding
gas
 Gaussian fits suggest a velocity
gradient in the emitting gas
Results: Radiative Transfer Analysis
• 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)
C18O & C17O
• Both molecules fitted simultaneously
• Two-component model needed for a proper fit
HCO+ & H13CO+
Results: Radiative Transfer Analysis
HCO+ & H13CO+
C18O & C17O
T = 35 K
nH2 = 105 cm-3
θ = 40 arcsec
T = 60 K
nH2 = 106 cm-3
θ = 9 arcsec
• 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
Results: Radial Structure of NGC 6334 I
• Radiative transfer model by Rolffs
et al. (2011)
• Model assumes a centrally
heated sphere with a powerlaw 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 from LVG analysis are remarkably consistent with the radial structure of
the envelope
Results: Origin of the Emission
0.1 pc
Results: Origin of the Emission
Results: Origin of the Emission
Results: Origin of the Emission
Results: Origin of the Emission
6.8x104 AU
No. 1, 2009
A MODEL FOR THE G31.41+0.31 HOT CORE
31
and the density is given by
ρSLS (r) = (P0 / 2πG)1/ 2 r −1 .
(3)
Inside the expansion wave (r < rew ), the infall velocity, whose
direction is radial, is given by
vSLS (r) =
1
(2πGP0 )1/ 2 t u(x),
2
having a free-fall behavior for r
ρSLS (r) =
(4)
s
s
0
r
R
e
x
t
p
Inner
envelope
rew . The density is
2
α(x),
π1/ 2 Gt 2
1.5 km s-1
+
s
0
2.6x104 AU
(5)
where x = 4(2πGP0 )−1/ 2 r t −2 is the similarity variable,
and u(x) and α(x) are nondimensional functions. With this
normalization the expansion wave is located at x = 1. The
self-similar variable, and the density and velocity functions are
related to those tabulated by McLaughlin & Pudritz (1997)
through x = 25/ 2 xMP , α(x) = 2−3 αMP (xMP ), and u(x) =
23/ 2 uMP (xMP ), where the subindex “MP” labels the McLaughlin
& Pudritz (1997) solution.
As noted by Osorio et al. (1999), the SLS collapse tends
to produce massive envelopes, since inside the radius of the
expansion wave only 3% of the mass is in the9central-3
star, while
t envelope.
97% is in the collapsing
8 due to9turbulent
-3
The velocity dispersion inside the envelope
a that Alfvén waves in the cloud
motions is obtained assuming
induce fluid motions with velocity amplitudes, δvtur , of the
order of the wave speed, δvtur
vA = (dP/ dρSLS )1/ 2 =
1/ 2
(P0 / ρSLS ) , where the magnitude of the magnetic field is B =
(4πP0 )1/ 2 . For random polarizations and random orientations
Outer
envelope
R
d
u
s
t
 Detection of the expanding envelope surrounding the hot core of NGC 6334 I
Figure 1. Geometry of the HMC. The plane containing the center of the core
and the line of sight is shown. Rdust and Rext are the inner and outer radii of the
envelope, respectively, p is the impact parameter of the line of sight, r is the
radius of a given point, s its coordinate along the line of sight, and −s0 , +s0 are
the coordinates of the envelope edges.
 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 / k = 1.6 x10 cm K
Ambient pressure exerted by the envelope, P / k = 10 – 10 cm K
 Thermal pressure drives the envelope expansion
the envelope, T(r), is self-consistently calculated from the total
luminosity using the condition of radiative equilibrium for outer
optically thin regions of the envelope, whereas for the inner
optically thick regions the temperature is calculated from the
standard diffusion approximation (see details in Osorio et al.
1999).
For temperatures
60 K and densities
105 cm−3 , the
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 N2H+ abundance.
 For [CO] / [H2] abundances of ~ 10-4, N2H+ is destroyed through reactions with CO,
forming HCO+ in the process
Results: Chemical Modeling
• Given that the production of HCO+ and N2H+ 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;
http://kida.obs.u-bordeaux1.fr)
Results: Cosmic Ray Ionization Rates
Component
Radiative transfer model
nH2 [cm-3]
[HCO+] / [N2H+]
Outer Envelope
105
6
Inner Envelope
106
23
Nahoon / KIDA model
ζ [s-1]
[HCO+] / [N2H+]
• Grid of density and cosmic ray (CR) ionization rates
 Estimate ζ for NGC 6334 I given the derived physical parameters from LVG analysis
T = 35 K
H
T = 60 K
H
Results: Cosmic Ray Ionization Rates
Component
Radiative transfer model
Nahoon / KIDA model
nH2 [cm-3]
[HCO+] / [N2H+]
ζ [s-1]
[HCO+] / [N2H+]
Outer Envelope
105
6
2.0 x 10-16
5
Inner Envelope
106
23
8.5 x 10-17
19
• Grid of density and cosmic ray (CR) ionization rates
 Estimate ζ for NGC 6334 I given the derived physical parameters from LVG analysis
T = 60 K
T = 35 K
Estimate ζ
Estimate ζ
Fix density
Fix density
H
H
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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