The Effect of High Energy Photons on Planet-Forming Disks inthe Orion Nebula

Lauren Bearden, Colgate University ’14, Daniel DeLuzio,Haverford College ’15, Joshua Reding, Colgate University

’15, and Brandon Zaccardo, Colgate University ‘15Advisor: Professor Jeff Bary, Colgate University

We undertook spectral analysis of fourteen protoplanetary disks(proplyds) in the Orion Nebula in order to detect the presence ofphotoevaporation due to the nearby Trapezium stars (Ori A, B, C,and D). The proplyd data was taken using the SpeX spectrograph atthe Infrared Telescope Facility (IRTF) on Mauna Kea in Hawaii,which measures in the 0.8 to 5.4 μm range. Using the programSpexTool, we extracted spectra from the images of these proplydsin order to detect molecular hydrogen vibrational emission atwavelengths 2.1218 μm and 2.4066 μm. After subtracting abnormalnebular emissions, we determined the flux of these features usingIRAF.  We found that the flux of the hydrogen features in theproplyds exceed those of the nebula, which indicates thatphotoevaporation is, in fact, taking place in these proplyds.

Introduction

The Orion Nebula provides a very interesting landscape for studying early star formation given the presence of the four O and B-type stars in the Trapezium, which give off massive amounts of radiation to the surrounding area.  This radiation has a significant effect on young stellar formation in the nebula, as it (heats the gas in the upper layers of the protoplanetary disks surrounding these new stars, potentially leading to the photoevaporation of the disk) causes photoevaporation in the protoplanetary disks (proplyds) surrounding these young stars.  Photoevaporation is the process by which gas and dust in a protoplanetary disk is ionized by energetic radiation in the near-UV (EUV) and far-UV (FUV) range of the electromagnetic spectrum, causing material to be released from the disk and greatly affecting its structure and possibly limiting the disk lifetime.

Figure 1: FUV photons are energetic enough to penetrate densely ionized gas regions and reach the accretion disk, which creates a flow of material off the disk and forms this characteristic teardrop shape.Surrounding EUV radiation helps to propagate this flow outwards, creating a bow shock at the front end of the proplyd facing the source of radiation.  [Photo credit Henney et al., 2000]

Aside from ionization of single atoms in the disks, theultraviolet radiation from the Trapezium also excites molecular hydrogen, the most abundant constituent of these disks.  The v=1-0 S(1) transition at 2.1218μm is particularly useful (as is the v=1-0 Q(1) transition at 2.4066μm) in determining the impact of the ultraviolet radiation on the proplyds and the presence of photoevaporative flows off the disk surfaces.  If it is determined that photoevaporation occurs in these proplyds, then we will have great insight into the dynamics of planet formation in high-energy areas as well as a better idea of the lifetimes of these young stars.

ObservationsTo attain the raw images of the select set of proplyds

for analysis, we used the spectrograph and imager Spex at the NASA Infrared Telescope Facility on Mauna Kea in Honolulu, Hawaii. SpeX is a medium-resolution cryogenic spectrograph and imager that diffracts the near-infrared light from a source and captures an image of its spectrum within the wavelength range of 0.8 and 5.5 microns (see Rayner & Wang 2002). The observed light passes through a thin 2 pixel slit of width 0”.3 and a length of 15”. In

order to eliminate background emission, the telescope nods the object between postion A ~ 3.5” and position B ~10.25” Through a series of grating, prisms, and mirrors, the light is diffracted into a spectrum and the image captured. The raw images are displayed as 6 bands, overlapping segments ofthe near-infrared spectra of the light passing through the slit.

Our hypothesis is that the radiation from the Trapeziumstars of the Orion Nebula is a critical factor in the H2 evaporation rate of the observed proplyds. With this in mind, it makes sense that a proplyd closer to the Trapezium stars would experience a higher flux of ultraviolet radiation than one further away, which will affect our future results for each proplyd. To get a rough idea of the proximity of our observed objects relative to the Trapezium cluster, we made a two-dimensional spacial distribution of our observed objects. Figure 2, generated using data from the SIMBAD Astronomical Database, illustrates this using coordinates of right ascension and declination.

Figure 2: Spatial Distribution of observed proplyds around the TrapeziumCluster of the Orion Nebula.

Data ReductionFrom the raw images taken with SpeX, we created

combined images of each separate object using the IDL program xspextool. The following procedure dictates what we

did to extract our data and discounts any other corrective process including calibration and elimination of dark currents (see Cushing, Vacca, & Rayner 2004). Each raw reading of an object at the same slit position was superimposed and normalized to give an average combined image. It is important to note the differences in airmass between some of the combined images, as well as ‘ghost images,’ which resulted from saturation and created some error in flux measurements. Xspextool then extracts a spectrum from each new image.

All of our later measurements, assumptions, and calculations were taken from these spectra. These spectra still contain nebular features, so to eliminate emission from the nebula and background sky, we initially took an image of the proplyd in the B position on the slit (~10.25”)and subtracted it from the image of the same object in the Aposition (~3.5”). However, in most cases, the nebula surrounding the proplyd was not uniform and therefore when we subtracted the images, we had residual subtraction features. Therefore, we did not use these images. Instead, to isolate the nebular emission from the proplyd emission, we took the original image at the A position and subtracted the B position of the star DQ Tau. This removed the background emission and dark currents, but did not isolate the proplyd spectrum from the nebular spectrum. Therefore, we took this image and extracted a spectrum from a point in the slit at about 4.75”, which was a sample of the nebula close to the proplyd. We took this to be the nebular spectrum. Finally we subtracted this from the image of proplyd and nebula, so we could get the proplyd spectrum. From here, both spectra were tellurically corrected for atmospheric absorption. An example of spectra with the telluric corrections is shown in Figure 2.

In order to telurically correct the spectra, we used another set of IDL programs in the spextool package, xtellcor and xtellcor_finish, which analyzed our combined spectra of a standard main sequence star. After we entered its known B and V magnitudes, we then created an absorption correction spectrum by fitting a normalized spectrum to the previously extracted one. This allowed us to isolate and remove atmospheric absorption features. The “absorption spectrum” was then divided out from any of the object spectra to cancel telluric emission using xtellcor_finish. From there, the orders of the corrected spectra were merged using the IDL program xmergeorders and final spectra were ready to be analyzed.

Though many of the proplyds varied in signal-to-noise, most spectra showed similar composition of elements, typically including a clear Paschen and Brackett series of hydrogen, some strong helium lines, sulfur III, and various other elements. Most of these lines were confirmed by authors Osterbrock and Rayner in their respective papers. Molecular hydrogen (H2) was of particular interest to us forour research, as it is an indicator of photoevaporation.

Figure 2 (above): This shows the isolated and nebular spectra corrected for atmospheric absorption features. Flux is in units ofergs/ s/cm2.

Most of the obvious features apparent in the spectrum of theproplyd were actually emission from the surrounding nebula. However, by carefully measuring the flux at particular wavelengths, specifically those characteristic of H2 emission (2.122μm and 2.406μm), we can verify the presence of molecular hydrogen within the disk. With a better understanding of H2 within and around the proplyd, we can continue research to calculate photoevaporation rates of this gas and other heavy elements from the proplyd.

AnalysisTo learn about what was really happening in the proplyd

system and to gain some insight into photoevaporative dynamics, we had to study the specific features in our spectra. We know that molecular hydrogen is usually a good indicator of photoevaporation in proplyds, so we measured important features such as the v = 1-0 S(1) feature at 2.122μm and the v = 1-0 Q(1) feature at 2.406μm.

The first step in this process was to identify the features in each spectrum. We conducted research to identifyfeatures that were relevant to our study. Many features wereidentified in Osterbrock and Rayner. We also used an online database that provided us with a list of atomic features within a wavelength range (van Hoof, 2012) to confirm those spectral features provided by the papers, and also identify features not mentioned in the literature. The database also included Einstein coefficients for each feature, which provide probability of the feature appearing in the spectrum.

After we identified the relevant spectral lines, we measured the strengths of those features using the IRAF program ‘onedspec’. We took two measurements for each emission feature: the flux, which is the area between the emission feature and the continuum, and the full width at half maximum, which we would use later to calculate the error. The software that we were using allowed us to zoom into the specific part of the spectrum that we were interested, and choose a point of continuum on either side of the feature in question. The IDL program then fit a

Gaussian curve to the feature and returned the measure the flux and FWHM for both the nebular and proplyd features.

We then took the measurements of the extracted nebular features and subtracted them from the respective proplyd features. This gave us a net flux, which indicated the amount of molecular hydrogen coming from only the protoplanetary disk. This value was important for us in finding whether there was molecular hydrogen coming from theproplyd, or if the molecular hydrogen that we detected was actually only coming from the nebula.

Error AnalysisIn order to calculate error for our spectral

measurements, we had to take into account both uncertaintiesin the continuum and in the flux measurements. We measured uncertainty in the continuum by identifying a relatively featureless area of the spectrum, and finding the variance of values in that area using a program written in IDL. In order to calculate the uncertainty in the flux measurements,we used the full width at half max (FWHM) that we obtained from the IRAF program when we measured the fluxes. From the FWHM, we obtained the standard deviation of the fluxes usingEquation 1

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FWHM =2σ 2ln2 =2.35σ Equation 1

where

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σ is the standard deviation. By using the standard deviation, we found the standard error using Equation 2:

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SE = σn Equation 2

where n is the value of data points that we had overall for a particular spectrum. In our spectra, this number was 4,179, denoting the number of pixels there were in each spectrum. We found the variance of the spectral lines, or the average squared deviation from the mean, for each relevant spectral line. We used Equation 3

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SD = 1N −1 (xi − x)2

i=1

N

∑Equation 3

where N is number of data points and x is the mean subtracted from the given data point. We combined the uncertainty calculated in the continuum and the uncertainty calculated in the spectral features using the equation

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Errorfinal = SDcontinuum* (fluxi)± SDfluxi=1

i=N

∑ .

Equation 4

In our error calculations, we must also consider the assumptions we made in order to analyze our results. First, in our spectral subtractions and analysis, we assumed that the nebula is uniform across the field and that the area underneath the protoplanetary disk that we measured is far enough away from the proplyd that we do not get any residualfeatures from it. This could affect our results and conclusions.

Though we are not finished analyzing error in our project, our preliminary evaluation seems to indicate that the error is not statistically significant, and that our results should be valid.

Results        When we subtracted the flux of the molecular hydrogen lines from the extracted nebular spectrum from the corresponding protoplanetary disk lines, we found that the net flux was generally positive. This means that when we compared the two spectra, molecular hydrogen had a stronger presence in the protoplanetary disk than in the nebula. Thissuggests that the infrared radiation that we measured is primarily coming from the proplyd. This may be an indicator of photevaporation from the protoplanetary disk. An example of the measurements of the proplyds is displayed in the Chart 1. This particular data is taken form the

protoplanetary disk labeled ‘p191-350’ in Figure 2. Notice that the flux of the protoplanetary disk lines are generallyabout an order of a magnitude greater than the correspondingline of the nebula. The difference between these is evidenceof the infrared radiation coming from the proplyd system, possibly indicating photoevaporation.

Future Studies        It is necessary to find the percent error in our results to find out if the differences in our fluxes are statistically significant in comparison. This uncertainty analysis will take into account both the spectral uncertainty and the flux uncertainty. Because the spectra that we are using range from .8-2.5 μm, it will also be of interest to not only continue analyzing the molecular hydrogen features in the spectra, but to also look more closely at other photoevaporative indicators such as HI.

Acknowledgements        We would like to thank the Keck Northeast Astronomy Consortium and the Natural Sciences Division at Colgate University for the opportunity to do this research, and our advisor, Professor Jeff Bary, for all of his help and support.

CitationsDarian, A. "NASA Infrared Telescope Facility." N.p., 6 Oct. 2010. Web. 20 Sept. 2013.Henney, W. J. & O’Dell, C. R., 1999, AJ, 118: 2350-2368Osterbrock, D. E., Shaw, R. A., & Veilleux, S., 1990, ApJ, 35: 561-573Rayner, J. T., Cushing, M. C., & Vacca, W. D., 2009, ApJS, 185: 289

Chart 1 (left): The measurements of wavelength, flux, and full width at half max are displayed. The data for the top 2