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. =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 (1.45-2.45{mu}m), enabling us to detect 87 emission lines from rovibrationally excited molecular hydrogen (H_2_) 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 H_2_ and physical conditions within the PDR. The Orion Bar PDR is thought to consist of cooler high density clumps or filaments (T=50-250K, n_H_=10^5^-10^7^cm^-3^) embedded in a warmer lower density medium (T=250-1000K, n_H_=10^4^-10^5^cm^-3^). We fit a grid of constant temperature and density Cloudy models, which recreate the observed H_2_ level populations well, to constrain the temperature to a range of 600-650 K and the density to n_H_=2.5x10^3^-10^4^cm^-3^. The best-fit model gives T=625K and n_H_=5x10^3^cm^-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 (1.45-2.45{mu}m). 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: --------------------------------------------------------------- RA (ICRS) DE Designation(s) --------------------------------------------------------------- 05 35 19.73 -05 25 26.7 Orion Bar = NAME Orion Bright Bar --------------------------------------------------------------- File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 93 87 H2 lines observed in the Orion Bar -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 8 F8.6 um lambda [1.5/2.5] H_2_ 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 H_2_ 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) -------------------------------------------------------------------------------- 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 {Delta}J=-2, Q is {Delta}J=0,and O is {Delta}J=+2. Z denotes the upper J state. Note (4): Normalized to the 4-2 O(3) reference line flux F_r_ (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). Note (8): lnR=ln((N_u_/g_u_)/(N_r_/g_r_)). In a transition's upper state N_u_ divided by the quantum degeneracy g_u_, normalized to N_r_/g_r_ 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 N_u_ to the column density predicted by our best fit model N_m_ (Section 5.2), as shown in the bottom of Figure 3. -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Prepared by [AAS], Emmanuelle Perret [CDS] 07-Nov-2017