Spatial variation in temperature and density in the IC 63 PDR from $\rm H_{2}$ Spectroscopy

We have measured the gas temperature in the IC 63 photodissociation region (PDR) using the S(1) and S(5) pure rotation lines of molecular hydrogen with SOFIA/EXES. We divide the PDR into three regions for analysis based on the illumination from $\gamma$ Cas:"sunny,""ridge"and"shady."Constructing rotation diagrams for the different regions, we obtain temperatures of T$_{ex}$=$562^{+52}_{-43}$ K towards the"ridge"and T$_{ex}$=$495^{+28}_{-25}$ K in the"shady"side. The H$_2$ emission was not detected on the"sunny"side of the ridge, likely due to the photo-dissociation of H$_2$ in this gas. Our temperature values are lower than the value of T$_{ex}$=685$\pm$68 K using the S(1), S(3), and S(5) pure rotation lines, derived by Thi et al. (2009) using lower spatial-resolution ISO-SWS data at a different location of the IC 63 PDR. This difference indicates that the PDR is inhomogeneous and illustrates the need for high-resolution mapping of such regions to fully understand their physics. The detection of a temperature gradient correlated with the extinction into the cloud, points to the ability of using H$_2$ pure rotational line spectroscopy to map the gas temperature on small scales. We used a PDR model to estimate the FUV radiation and corresponding gas densities in IC 63. Our results shows the capability of SOFIA/EXES to resolve and provide detailed information on the temperature in such regions.


INTRODUCTION
Molecular hydrogen is readily detectable in photodissociation regions (PDRs; Nadeau et al. 1991;Timmermann et al. 1996;van Dishoeck 2004). Hydrogen transitions from H 2 , via H I to H II in PDRs from the exposure to far-ultra violet (FUV) radiation from O/B stars and the interstellar radiation field. Through photochemical reactions and photo-electric emission, both the chemistry and temperature balance of the PDRs is controlled by the FUV radiation (Röllig et al. 2007). It is therefore important to characterise the over-all as well as small scale structure of these regions.
In the regions directly affected by FUV radiation, the thermal balance is related to the radiative transfer of the UV photons. The FUV radiation is also responsible for the kinematics and chemical changes of these regions (Draine & Bertoldi 1999). The ejection of photoelectrons from the dust grains is the the process accountable for heating of the region (Bakes & Tielens 1994;Weingartner & Draine 2001). In addition to this process, the collisional de-excitation of H 2 molecules initially excited by UV photons is also a mechanism contributing to the heating (Sternberg & Dalgarno 1989). Towards the higher density regimes, gas-grain collisions may also heat the gas. On the other hand, the fine-structure lines of neutral atoms or of singly ionized species provide the cooling of the outer layers and CO rotational lines cool the predominantly molecular inner regions (Allers et al. 2005).
The observations of rotational and vibrational lines of molecular hydrogen is a useful tool to diagnose the physical properties of the PDRs. As investigated by Allers et al. (2005) towards Orion bar, the low critical density of the ground-state rotational transitions makes ratios of mid-IR lines good probes of the temperature in the layers where they arise. In PDRs with n(H) ∼ 10 4 − 10 5 cm −3 , the lower rotational levels of H 2 (i.e. J=2-0, J=3-1) are maintained in thermal equilibrium by collisions (Habart et al. 2005). The critical density of n(H) > 10 5 cm −3 is needed for higher transitions (i.e. J=5-3, J=7-5). The populations of these levels can be a good tracer of gas temperature.
We investigated the pure rotational molecular hydrogen line emission in the PDR IC 63. This is a small, but well-studied (at d≈200 pc; Karr et al. 2005), PDR irradiated by the (FUV) light from γ Cas, a B0.5 IV type star located ∼1.3 pc projected distance from the cloud. This is the nearest and well explored H II system to investigate the PDR properties. The system contains nebaule IC 59 and IC 63 irradiated by γ Cas. The projected direction of FUV radiation from γ Cas on IC 63 is from southwest to northeast.
The goal of SOFIA proposal 07 0040 (PI: Soam, A.) was to quantify the difference in gas temperature in gas directly exposed to the light from γ Cas, and gas in the shade of a molecular clump within the PDR. Here, we present results of observations of the S(1) and S(5) lines of H 2 . We compare our SOFIA/EXES (Stratospheric Observatory for Infrared Astronomy/Echelon-Cross-Echelle Spectrograph) (Richter et al. 2018) findings with the results of Thi et al. (2009) toward a different location of the IC 63 PDR. As is often the case in the ISM (Ehrenfreund et al. 1998), Thi et al. (2009, using observations of S(0), S(1), S(3), and S(5) from the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO), found a twotemperature structure in the H 2 excitation. The two lowest rotation transitions found a low temperature, typical of the cold neutral medium (McKee & Ostriker 1977), in their case T ex =106±11 K, while their three highest transitions, yield a significantly higher temperature of T ex =685±68K.
Such high(er) excitation temperatures in the high J lines of H 2 are commonly seen in FUV absorption spectroscopy and has been variously attributed to the effects of UV pumping via the Lyman and Werner band excitations (e.g. Jura 1975) and to collisional heating, including through shocks. Andrews et al. (2018) investigated the physical conditions of UV-illuminated surface of IC 63. Using archival Spitzer IRS data of molecular hydrogen in IC 63, they constructed the excitation diagram to find gas temperature in IC 63 PDR. Assuming that lines are optically thin, adopting an ortho-to-para ratio of three, and using energies and Einstein coefficients from Rosenthal et al. (2000), they found two temperature components in IC 63 PDR.
The observed correlation between the column densities of the CH + ion (whose formation, via the reaction of C + with H 2 , includes a ∼4600 K activation barrier; Millar et al. 1991) and H 2 J=3 and J=5 (e.g. Lambert & Danks 1986) indicates that collisional excitation does contribute to the high-J population of H 2 .
In PDRs, FUV pumping, photo-dissociation, and photo-electric emission are critical processes.
On the PDR outer boundary, hydrogen exists mainly in atomic form because most of the molecular hydrogen is photo-dissociated by the high energy UV photons (Thi et al. 2009). As we move into, or behind, high density/extinction regions inside the PDR, the strength of the UV radiation is reduced by dust extinction, which, together with line self-shielding, reduces the photo-dissociation in comparison to H 2 formation on the dust grain surface and the molecular fraction increases (Habart et al. 2004(Habart et al. , 2011. While the cut-off for photoelectric heating is not at quite as short a wavelength as for H 2 destruction (1550Å for an assumed work function of 8 eV; Weingartner et al. 2006), the extinction at such wavelengths is still very steep. As the shielding increases, we would therefore expect the gas temperature to drop. In an inhomogeneous medium where significant shading occurs due to opaque clumps, a (variable) two temperature structure would be expected, corresponding to gas experiencing limited or significant FUV extinction. Given the size and location of the Thi et al. (2009) ISO apertures (i.e. ≈ 25 or 0.025 pc at the distance of IC 63), it is conceivable that their twotemperature result originate from a mixture of gas at the front and back of the H → H 2 transition over their apertures ( Figure 1). Therefore, we set out to investigate the variation of the temperatures (if present) throughout the PDR, using higher spatial resolution observations than those from ISO/SWS. With its narrow slit, EXES on SOFIA has the capability to produce such higher spectral resolution data for these investigations.
The goal of this paper is to investigate gas temperatures in those regions of IC 63 PDR which are directly illuminated by and in shade of UV radiation from γ Cas. Section 2 presents the details of SOFIA/EXES observations. Section 3 presents our results and analysis performed. Section 4 shows the detailed discussion of our results and section 5 concludes our findings. We observed IC 63 with SOFIA using the EXES instrument in its high-medium mode (Richter et al. 2018), selecting the highest operable cross dispersion order to maximize the spatial length of the slit. For the H 2 S(1)  Spitzer IRAC image (taken from Spitzer archive). The larger solid green rectangle is the ISO/SWS S(1) (17.0µm) aperture and the red solid rectangle is the S(5) (6.9µm) aperture (Thi et al. 2009). The purple dot-dash line shows the zoomed in image in the right panel and the yellow rectangles show the three regions observed by SOFIA/EXES. The direction to γ Cas is shown as well. The right panel shows the zoomed-in view of the three extraction regions and the EXES slit positions over the 8.0µm IRAC image. The SOFIA/EXES S(1) aperture is the larger solid green rectangles and the S(5) aperture is the smaller solid red rectangles. The slits swept clockwise during observations. We have defined region 1 as "shady", region 2 as "ridge" and region 3 as "sunny" and they are shown as translucent yellow rectangles.
observations, we observed at 587 cm −1 in 3 rd order, with a slit length of 19 (0.0184 pc assuming a distance of ≈200 pc). For H 2 S(5), we observed at 1447 cm −1 in 8 th order, with a slit length of 10 (0.0097 pc). All observations used the 3.2 (0.0031 pc) slit width, which provides R ∼60000 resolution, or ∼ 5 km s −1 (measured with laboratory gas cell data prior to flight (EXES PI team, private communication)).
We observed IC 63 using off-source nodding. We selected an off-region of blank sky 30 East of the central slit coordinate (Table 1) and nodded at intervals of 60 s in order to subtract telluric emission and thermal background from the on-source integration.
The spatial resolution of the observations is approximately 3.7 for both wavelengths, which is taken from the 50% encircled energies reported in the SOFIA observatory characterization (Temi et al. 2018). The telescope was guided using the Focal Plane Imager Plus (FPI+) guider camera on a visually bright star offset from IC 63. This mode of guiding is astrometrically accurate to 0.41 with each new target acquisition and with a median pointing stability of 0.17 following acquisition (Temi et al. 2018).
Flux calibration was done by scaling the data to spectral flat fields with the instrument's external blackbody source before each observation. The resulting absolute flux calibration is good to 25%. The relative flux calibration between S(1) and S(5) is 12.5% but any systematic differences in the relative or the absolute flux calibration are repeatable to 2% (EXES PI team, private communication).
The rotation of field (ROT) for SOFIA observations is determined by the heading of the airplane but can be held fixed for short periods by the telescope. Because EXES does not have a field rotator, as some SOFIA other instruments, the slit position angle (PA) changed by discrete amounts every 6 minutes during the observations. We executed individual nodded files within that interval, typically obtaining 4 nod pairs per file and per discrete PA. Table 1 reports the starting and ending slit position angles, which are also visualized in Figure 1.
Data were reduced using with the SOFIA Redux pipeline (Clarke et al. 2015), including steps for spike removal, nod-subtraction, flat fielding, flux calibration, order rectification, and coadding of nodded pairs. The wavelength scales were calibrated by matching the sky emission lines within the spectral settings to their values in the HITRAN database (Gordon et al. 2017). The uncertainty in the wavelength solution is estimated to be ∼ 0.3 km s −1 .
Because of the changing slit angle during observations, we employed a customized procedure for combining the data. We defined three 5 -width intervals of RA, as shown in Figure 1. The RA centers of the regions were 1) 00:59:00.61, 00:58:59.93 and 3) 00:58:59.25. We used the central slit coordinate and position angle of each file to assign and extract the appropriate parts of the 2-D spectrum to regions 1, 2 and 3, weighting each extracted spatial row by the file's integration time. Because there were up to 20 nodded files and different position angles used during each night's observation, we only list the starting and ending position angle in Table 1.
Our observations of the H 2 S(1) and S(5) lines with the narrower EXES slit toward IC 63 are located in a region chosen to provide as narrow a PDR ridge as possible, somewhat north of the region observed by Thi et al. (2009). The location of the EXES slits and the Thi et al. (2009) Thi et al. (2009) while the observed regions by EXES are shown with semi-transparent yellow rectangles. We designated these regions, labeled as 1, 2, and 3 in the right panel of Figure 1, as the "shade", "ridge", and "sunny" sides, respectively, of the PDR. The right panel of the Figure 1 also shows the EXES slit rotation pattern and actual extraction regions of S(1) and S(5) emission shown with green and red rectangles, respectively.
The simple vertical RA bars we used for regions were well-motivated by the 2.12µm ro-vibrational H 2 emis-sion and 8.0µm Spitzer/IRAC emission (Figure 1). Because of the exploratory nature of the observations, most of the S(1) was acquired in regions that were close but did not strictly overlap the S(5) coverage. But the full integration time/spatial extent of the S(1) line was needed to obtain a useful line detection. This might mean that the temperature gradient reported here could be mimicked by an inopportune brightness gradient between the total S(1) footprint and the S(5) footprint. However, the 2.12µm ro-vibrational emission and Spitzer/IRAC image do not suggest such a gradient.

Near-IR imaging of ro-vibrational line
We also adopted H 2 1-0 S(1) line observations of IC 63 at 2.122µm from Andersson et al. (2013) for our analysis in this work. They acquired this data from the WIRCam instrument (Puget et al. 2004) at the Canada-France-Hawaii Telescope (CFHT) in queue observing mode during 2008 August 12--September 14. In these observations, Andersson et al. (2013) used Ks filter to estimate the continuum contributions to the nebular emission because the narrow-band continuum filter associated with the H 2 1-0 S(1) filter was not installed at the time of these observations. The data was reduced using observatory pipeline. The photometric calibration of the data was achieved using several hundred 2MASS (Skrutskie et al. 2006) stars in the observed field. More details of observations, data reduction, and calibration can be seen in the paper by Andersson et al. (2013). Figure 2 shows the spectra for the S(1) and S(5) lines. We measured flux values for the S(1) and S(5) lines in the "ridge" and "shady" regions, but we detected no emission in the "sunny" region (Table 2). We calculated the flux values shown in Table 2 by calculating the area of the Gaussian fits of the line spectra shown in Figure 2.The line widths used to calculate these areas are shown in Table 2. We compared this with the direct line integration method and found agreeing flux values. The flux uncertainties are derived from the uncertainties of the fitted Gaussian parameters. The integrated intensity of the S(1) line in the "shady" region is a factor of 2 larger than that in the "ridge" region which indicates warmer gas in the "ridge" region. The integrated intensity of the S(5) line is approximately the same in both regions. The S(1) integrated intensity in the "ridge" region agrees with the S(1) ISO SWS intensity from Thi et al. (2009) while the ISO SWS S(5) value is approximately twice the intensity of the EXES S(5) intensity values in both regions.   Figure 1 for explanation of regions b. Einstein A-coefficients from Wolniewicz et al. (1998). c. Line width values were calculated from inspection of each line profile individually and may differ from values given by the fits in Figure 2 We calculated the population of the upper level, N u , using the equation:

Rotation diagram
where I ul is the total intensity, A ul is the Einstein Acoefficient, and ν ul the frequency of the transitions from the upper to lower levels. We constructed an H 2 excitation diagram (see Figure 3) by plotting the log of the population of the upper level divided by the upper level degeneracy and nuclear spin degeneracy (g u , g I respectively) versus the temperatures of the upper energy levels which are 1015 K for S(1) and 4586 K for S(5). The excitation temperature at local thermodynamic equilibrium, T ex , can be related to the population of the upper level, N u , by the equation: where E ul is the energy of the transition expressed in Kelvin, N(H 2 ) is the H 2 column density and Q(T ex ) is the partition function. In the rotational excitation diagram shown in Figure 3, we overplottetd EXES data (red solid circles) with the data taken from Thi et al.
(black filled squares). Based on these two lines, we derive excitation temperatures of T ridge =562 +52 −43 K and T shade =495 +28 −25 K. The errors given here are 1σ level. This shows that the ridge of IC 63 seen with EXES observations is at a similar temperature (within uncertainties) as the high temperature reported by Thi et al. (2009) (T ex =685±68K). However, our results shows that the shady region of IC 63 is somewhat cooler than their reported temperature. The inferred H 2 column density from the excitation temperatures of the ridge and shady regions are 9.8×10 19 cm −2 and 3.6 × 10 20 cm −2 , respectively. We could not derive excitation temperature in the sunny side of the cloud as we did not detect either the S(1) or S(5) lines there (see Figure 2).
As shown in left panel of Figure 1, the SWS footprint is significantly larger than the EXES slit width and located further south -and therefore closer the to illuminating star γ Cas. Because of the area of the SWS aperture, it encompasses gas both on the near and far side of the fluorescent ridge -as seen from γ Cas. Based on the location and size differences we hypothesize that the warm gas probed by the ISO/SWS measurement may possibly represent higher illumination regions, or a more "sunny" location and that the EXES observation probe gas with a higher extinction toward the star or a more "shady" spot if the actual distance of ISO slit is lesser than that of EXES slit from γ Cas. However, the projected distances from γ Cas of the ISO and EXES slits are almost the same. The cold temperature (T ex =106±11 K) seen in the ISO data would then be tracing well-shielded gas behind a dense clump. This hypothesis can be tested by extending the wavelength and spatial coverage to probe both the H 2 S(1)/S(0) temperature in the EXES location and by isolating more spatial locations on either side of the PDR ridge. We are currently pursuing such observations.

PDR modeling
We used the PDR toolbox (Kaufman et al. 2006;Pound & Wolfire 2008) to model our observed line ratio H 2 1-0 S(1)/0-0 S(1). This model takes into account both thermal and UV excitation for the H 2 level population. We have plotted the predicted values for different values of FUV radiation field intensity (G 0 ) and volume density (n) in Figure 4, where we have also overplotted the observed line ratios. The upper panel of Figure 4 shows the observed values over-plotted for the ridge and shade sides using red and black lines, respectively. The shaded regions show 3σ uncertainties limits in the mean values. We find values of G 0 , in shade and ridge of ∼ 200 and ∼ 400 in Habing units, respectively. Using conversion from Habing units to Draine unit (Draine = 1.7×Habing) (Wolfire & Kaufman 2011), these values are 340 and 680 Draine units in shade and ridge, respectively. The value of G 0 in ridge is consistent to the G 0 value of 650 Draine units reported by Jansen et al. (1994) in IC 63.
The dashed box labeled as 1 in the upper panel of Figure 4 clearly show different G 0 values for the ridge and shade sides (red and black solid lines). This region is zoomed-in in the lower panel of he figure where the G 0 gap is indicated by double headed arrows. We used this difference in G 0 to estimate the gas density in the PDR by adapting the radiative transfer equation: where G 0shade and G 0ridge are the model returned values of G 0 for the line-ratios observed in the shade and ridge sides of IC 63 PDR, respectively. τ is optical depth of the region for the wavelength determining the line ratio (i.e. gas temperature). Using G 0 values in eq. 3, we estimated an optical depth of 0.4. In PDRs, when FUV photons are absorbed, the grain may be ionized (photoelectric effect) and part of the photon energy is carried away by the electron, heating the gas. Assuming a work function of 8 eV (Weingartner & Jordan 2008) corresponds to a wavelength of ∼1550 A we can estimated the gas density in the PDR using the optical depth estimated above. Using relation A 1550 A /A v = 2.64 from Whittet (2003), and A 1550 A = 1.086 × τ , we estimated A v of ∼ 0.16. This Figure 3. Rotational diagram created with ISO-SWS (solid black rectangles) and EXES data (solid red circles). The two temperatures seen by ISO/SWS data (Thi et al. 2009) are labeled and the temperatures of ridge and shady regions estimated with EXES data also shown with the text in red and blue colors.
value of A v and total-to-selective extinction (R v = 3.1) together with the relationship N(H) = 1.87 × 10 21 A v and N(H 2 ) = 0.5N(H) from Bohlin et al. (1978), yields a column density N(H 2 ) of 2.9 × 10 20 cm −2 . Assuming a constant space density and that the absorption takes place over a region of projected length L we can estimate the space density. If we assume L to be the EXES slit size of the H 2 S(1) observations i.e. 19 shown in Figure  1, we estimate a volume density n(H 2 ) of 5 × 10 3 cm −3 .
We also estimated column densities in shade (3.6 × 10 20 cm −2 ) and ridge (9.8 × 10 19 cm −2 ) regions of IC 63 from the rotation diagram (Sec. 3.1). Using these values over a projected length L at the location of EXES slit, we derived space densities of 6.2 × 10 3 cm −3 and 1.7 × 10 3 cm −3 in shade and ridge sides, respectively. These density values are consistent with our findings when assuming heating due to the photoelectric effect. Jansen et al. (1994) presented far-infrared spectroscopic measurements of IC 63. They measured line ratios of CO, HCO + , HCN, CS and HCHO suggesting that cloud is warm with a temperature ≈50 K, and the density of the gas is 5 × 10 4 cm −3 . This value is ten times higher than our estimation above. It might be possible that the values reported in Jansen et al. (1994) are biased towards denser clumps.
As it can be noticed in Figure 4, the modeled H 2 1-0 S(1)/0-0 S(1) line ratio gives a completely different answer for the density. The density value obtained here is of the order of 2 × 10 2 cm −3 . It is also noticed by Thi et al. (2009) that the H 2 S(3)/S(1) line ratio gives a completely different answer for FUV radiation strength and density. Thi et al. (2009) uses the explanation given by Bertoldi (1997) and Allers et al. (2005) to counter these discrepancies. It is suggested that either the H 2 formation rate on grains needs to be larger or the photoelectric heating efficiency needs to be increased at high temperatures, shifting the transition zone of H → H 2 closer to the warm edge of PDRs. High photoelectric heating efficiencies have been calculated by Weingartner & Draine (1999) based on an enhanced dust-to-gas ratio in the PDR due to gas-grain drift. We agree with the findings and possible causes explained by Thi et al. (2009). The photoelectric heating might be underestimated at hotter regions which may cause in discrepancies in the density estimation. Also, we did not include a detailed analysis on H 2 formation rate in this work because that is beyond the purpose of this paper. An underestimation in assuming the rate might also cause the discrepancies between model and observed densities.

DISCUSSION
Our results show that SOFIA/EXES is capable of investigating the spatial variation in temperatures in such regions. Although we do not see a big difference from temperatures values of Thi et al. (2009), we are capable of better resolving the spatial locations in the IC 63 PDR. The spatial coverage of ISO data used by Thi et al. (2009) was ≈ 25 or 0.025 pc at the distance of IC 63. Andrews et al. (2018) investigated the physical conditions of UV-illuminated surface of IC 63 using Spitzer archival data of molecular hydrogen. They used the short and long wavelength (SL, 5.2-14.5 µm; LL, 14.0-38.0 µm) low resolution modules (R ∼ 60-130). The spatial coverage of SL mode was ≈ 0.8×1.0 arcmin 2 (i.e., 0.046 × 0.058 pc 2 at the distance of IC 63) and the spatial coverage of LL mode on the tip of IC 63 nebula was ≈ 2.8 × 2.4 arcmin 2 (i.e., 0.16 × 0.14 pc 2 ).
They found the cooler temperature (T=207±30 K) associated to higher column density of 2.3 × 10 20 cm −2 and the warmer component (T=740±47 K) is associated to lower density 9.7 × 10 17 cm −2 region. Their values are in good agreement with the hot component (i.e., T=685 ±68 K) of Thi et al. (2009). But these values are higher than what we found as hot component (i.e., T ridge =562 +52 −43 K) with SOFIA/EXES higher spatial resolution observations. This suggests that Thi et al. (2009) and Andrews et al. (2018) get an averaged value of temperature in the bigger aperture of the ISO/SWS and Spitzer/IRS, respectively. Whereas EXES observations are further capable of resolving temperature of PDRs. Soam et al. (2021) showed that the collisional disalignment rate of the dust grains causing the observed polarization follow a bifurcated relation with respect to the gas density. The two observed sequences in derived disalignment rate correspond to lines of sight in front and behind gas clumps, as seen in the HCO + (J=1-0) map of the cloud. A two temperature structure in the gas, caused by variable internal shading by molecular clumps, would explain this observed bifurcation.
We used PDR modeling in Section 3.2 to estimate the density of the gas adapting the radiative transfer equation and assuming that the heating is due to the photo-electric effect. The density of gas is found to be 5 × 10 3 cm −3 . Our estimation of density is consistent to the value reported by Jansen et al. (1994). Habart et al. (2011) in their investigation on number of PDRs stated that for the physical conditions prevailing in our PDRs, the heating is mainly due to the photoelectric effect. Authors also report that for a range of H 2 formation rates and densities in various PDRs, the gas temperature vs. PDR depth profile remains insensitive to the fraction of molecular hydrogen. However, a higher formation rate of molecular hydrogen may shift the photodissociation front towards higher temperatures.
The non-detection of H 2 pure-rotational emission in the "sunny" part of the nebula is, in retrospect, not surprising. The locations of the "sunny" and "shade" regions were chosen based on mid/far infrared contiunuum and and H 2 v=1-0 S(1) line imaging. To allow for the presence of H 2 not detected by the fluorescent emission -possibly as a result of a highly clumpy medium on small scales -("sunny") locations star-ward of the fluorescent ridge were included in our observations. However, given the sharp transition expected from atomic to molecular form of hydrogen, caused by the self-shielding of the molecule (e.g. Federman et al. 1979) the nondetection of H 2 in the "sunny" regions is consistent with this sharp transition and indicates that the PDR is not very clumpy.

CONCLUSIONS
We have re-investigated the temperature in the IC 63 PDR using pure-rotational molecular hydrogen observations of S(1) and S(5) lines using SOFIA/EXES observations. A similar investigation was done by Thi et al. (2009) in this nebulae using lower spatial resolution ISO/SWS observations. The higher spatial resolution of EXES over SWS enables us to spatially resolve temperature in this PDR. Thi et al. (2009) reported two components of gas using S(0), S(1), S(3), and S(5) pure rotational lines of molecular hydrogen. The warm component is found to be at T ex =106±11 K and hot gas component is seen at T ex =685±68 K. We divided IC 63 PDR into "shade", "ridge" and "sunny" sides for our investigation. By constructing rotation diagram using S(1) and S(5) lines data from EXES, we obtained temperature of T ex =562 +52 −43 K towards the "ridge" and T ex =495 +28 −25 K in "shady" side. The PDR toolbox code was used to model the line ratios of detected emission in H 2 1-0 S(1) and 0-0 S(1) transitions. Our model suggests a lower value of FUV radiation (G 0 ) in the shade as compared the one in ridge. We used this damping in FUV radiation to estimate the optical depth of the PDR. Our results emphasize that SOFIA/EXES has the capability to resolve and provide detailed information on gas temperature and density of such regions. We are attempting to expand this work through observations of the S(2) and S(4) transitions from IC 63.