J/ApJ/838/152 Deep NIR spectrum of the Orion Bar PDR (Kaplan+, 2017)
Excitation of molecular hydrogen in the Orion Bar Photodissociation Region
from a deep near-infrared IGRINS spectrum.
Kaplan K.F., Dinerstein H.L., Oh H., Mace G.N., Kim H., Sokal K.R.,
Pavel M.D., Lee S., Pak S., Park C., Oh J.S., Jaffe D.T.
<Astrophys. J., 838, 152-152 (2017)>
=2017ApJ...838..152K 2017ApJ...838..152K (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Interstellar medium ; Spectra, infrared
Keywords: infrared: ISM; ISM: individual objects: Orion Bar;
ISM: molecules; photon-dominated region PDR; techniques: spectroscopic
Abstract:
We present a deep near-infrared spectrum of the Orion Bar
Photodissociation Region (PDR) taken with the Immersion Grating
INfrared Spectrometer (IGRINS) on the 2.7m telescope at the McDonald
Observatory. IGRINS has high spectral resolution (R∼45000) and
instantaneous broad wavelength coverage (1.45-2.45µm), enabling us
to detect 87 emission lines from rovibrationally excited molecular
hydrogen (H2) that arise from transitions out of 69 upper
rovibration levels of the electronic ground state. These levels cover
a large range of rotational and vibrational quantum numbers and
excitation energies, making them excellent probes of the excitation
mechanisms of H2 and physical conditions within the PDR. The Orion
Bar PDR is thought to consist of cooler high density clumps or
filaments (T=50-250K, nH=105-107cm-3) embedded in a warmer
lower density medium (T=250-1000K, nH=104-105cm-3). We fit a
grid of constant temperature and density Cloudy models, which recreate
the observed H2 level populations well, to constrain the temperature
to a range of 600-650 K and the density to nH=2.5x103-104cm-3.
The best-fit model gives T=625K and nH=5x103cm-3. This
well-constrained warm temperature is consistent with kinetic
temperatures found by other studies for the Orion Bar's lower density
medium. However, the range of densities well fit by the model grid is
marginally lower than those reported by other studies. We could be
observing lower density gas than the surrounding medium, or perhaps a
density-sensitive parameter in our models is not properly estimated.
Description:
The data were taken with the IGRINS on the 2.7m Harlan J. Smith
Telescope at the McDonald Observatory on the night of 2014 October 24 UT;
R∼45000 or 7.5km/s in two separate H- and K-band channels (1.45-2.45µm).
Figure 1 shows the finder chart and the IGRINS slit position and angle
superposed on the Orion Bar. The center of the slit was positioned at
05:35:19.73,-05:25:26.7 (J2000).
Objects:
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RA (ICRS) DE Designation(s)
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05 35 19.73 -05 25 26.7 Orion Bar = NAME Orion Bright Bar
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 93 87 H2 lines observed in the Orion Bar
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See also:
J/A+A/496/153 : Molecular hydrogen flows along Ori A cloud (Davis+, 2009)
J/ApJ/786/29 : Catalog of distances to molecular clouds (Schlafly+, 2014)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 8 F8.6 um lambda [1.5/2.5] H2 line vacuum wavelength (1)
10- 15 F6.2 10-6um Dellam [-21.5/9.7] Difference between lambda and
observed centroid (2)
17- 25 A9 --- Line H2 line rovibrational identifier (3)
27- 32 F6.3 [-] logFi/Fr [-1.7/1.3] Log normalized line flux (4)
34- 38 F5.3 [-] E_logFi/Fr [0/0.1] Upper uncertainty in logFi/Fr
40- 44 F5.3 [-] e_logFi/Fr [0/0.2] Lower uncertainty in logFi/Fr
46- 51 F6.1 --- S/N [4/1032] Signal-to-Noise (5)
53- 54 I2 --- vu [1/10] Transition upper vibrational state
56- 57 I2 --- Ju [0/13] Transition upper rotational state
59- 63 I5 K Eu/k [6149/45317] Energy of upper state (6)
65- 69 F5.2 [s-1] logAul [-8/-5.5] Log rovibrational radiative
transition probability (7)
71- 76 F6.3 [-] lnR [-5.2/4] Natural log column density (8)
78- 82 F5.3 [-] E_lnR [0.001/0.3] Upper uncertainty in lnR
84- 88 F5.3 [-] e_lnR [0.001/0.3] Lower uncertainty in lnR
90- 93 F4.2 --- Nu/Nm [0.3/2] Ratio observed column density (9)
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Note (1): Calculated from the energy levels in Komasa J., Piszczatowski K.,
Lach G. et al 2011 J. Chem. Theory Comp. 7, 3105.
See Section 3.4 for more details.
Note (2): The observed line centroid wavelength (in the Orion Bar rest frame)
minus the expected theoretical line wavelength calculated from the
level energies in Komasa J., Piszczatowski K., Lach G. et al.
2011 J. Chem. Theory Comp. 7, 3105.
Note (3): In spectroscopic notation in the format "W-X Y(Z)." W and X denote
the transition's upper and lower v states. Y denotes the change in J
where S is ΔJ=-2, Q is ΔJ=0,and O is ΔJ=+2.
Z denotes the upper J state.
Note (4): Normalized to the 4-2 O(3) reference line flux Fr (Section 3.5).
Note (5): For the line flux (Section 3.5).
Note (6): Above the ground (v=0, J=0) divided by the Boltzmann constant k
to convert the energies into temperature units (Section 4.3).
Note (7): From Wolniewicz+ (1998ApJS..115..293W 1998ApJS..115..293W).
Note (8): lnR=ln((Nu/gu)/(Nr/gr)). In a transition's upper state
Nu divided by the quantum degeneracy gu, normalized to
Nr/gr for the reference line 4-2 O(3) (Section 4.2 and
Section 4.3). This is the value plotted in the excitation diagram
shown in Figure 3.
Note (9): The ratio of the observed column density of the transition's upper
state Nu to the column density predicted by our best fit model
Nm (Section 5.2), as shown in the bottom of Figure 3.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 07-Nov-2017