Identification of Polycyclic Aromatic Hydrocarbons in Interstellar Clouds

Introduction The diffuse interstellar bands (DIBs) are a phenomenon found in astronomical absorption spectra in which the origin of the absorption lines is unknown. Among the postulated carriers for the DIBs are polycyclic aromatic hydrocarbons (PAHs), long chains or groupings of aromatic rings.1,3 This hypothesis stems from the sharp increase in absorptivity around 2175 angstroms (Å), a wavelength at which many PAHs have a strong absorption line (Fig. 1). PAHs have been shown to have strong absorption in the visible wavelength region (4000-7000 Å) and their cations have been shown to absorb in the near infrared. In particular, a study done by Fulara, Jakobi, and Maier showed that buckminsterfullerene cation had a doublet feature that absorbed around 9500 and 9600 Å respectively.2

Results The ‘telluric’ routine satisfactorily removed the strong water vapor absorption lines in the region containing buckminsterfullerene. A potential absorption feature was detected at 9577 Å in multiple sightlines, but no feature was detected at 9632 Å in any of the sightlines. In order to verify that any DIBs found in this region originated from buckminsterfullerene, both features needed to be present.

We could not conclude with a high degree of certainty that the feature present at 9577 Å is a DIB rather than a procedural remnant from the telluric correction that was used to remove the water vapor. The typical equivalent width of the ‘window’ in which the detected feature resides was approximately 620 mÅ whereas the detected feature had a typical equivalent width of 410 mÅ. A feature of the same strength needs to be observed at varying degrees of telluric correction in different nightly sums.

Amanda Faint1, & Professor Julie Dahlstrom2 1 Carthage College Department of Chemistry, 2 Carthage College Department of Physics and Astronomy

Initial Observations The spectra for the program was obtained from 1999 to 2007 with the 3.5 m telescope and the Astrophysical Research Consortium echelle spectrograph (ARCES) at Apache Point Observatory. The ARCES employs an echelle grating, two cross-dispersing prisms, and a SITe CCD detector with a 2048 x 2048 µpixels. Each ARCES spectrum provides complete spectral coverage from about 3700 to 10000 Å at a resolving power of R = 38,000.4 The typical signal-to-noise ratio at the wavelength of peak instrumental sensitivity was S/N > 1000 though lower in the 9500 Å region.

A gap appears in between the strong water vapor absorption lines present in the considered region within which the two proposed buckminsterfullerene peaks reside. Without performing a correction it is difficult to discern if there are any underlying peaks (Fig. 3).

Near Infrared Spectrum

The near infrared region of ground based astronomical spectra contain strong contamination from water vapor absorption (Fig. 2). Contamination from water vapor absorption may be minimized by collecting observation data when the air is relatively dry without impacting the strength of underlying stellar or interstellar features.

Figure 5: Multiplot of uncorrected, corrected, and comparison spectra for one of the analyzed stars, 9577 Å and 9632 Å highlighted

Research Objectives:

1.! To perform a correction for the strong absorption of water vapor in the near infrared

2.! To verify the presence of absorption peaks at 9577 and 9632 Å corresponding to buckminsterfullerene2

Acknowledgements

•! Carthage SURE program •! Dr. Julie Dahlstrom •! Dr. Donald G. York and Dr. L.M. Hobbs, University of Chicago •! Dr. Benjamin J. McCall, University of Illinois at Urbana-Champaign •! Dr. Theodore P. Snow, University of Colorado •! Carthage Department of Physics & Astronomy

References [1]Allamandola, L. J., A. G. G. M. Tielens, and J. R. Barker. "Interstellar Polycyclic Aromatic

Hydrocarbons: The Infrared Emission Bands, the Excitation/emission Mechanism, and the Astrophysical Implications." The Astrophysical Journal Supplement Series 71 (1989): 733-75. Print.

[2]Jenniskens, P., G. Mulas, I. Porceddu, and P. Benvenuti. "Diffuse Interstellar Band near 9600 A: Not Due to C60+ Yet." The Journal of Astronomy and Astrophysics (1997). Print.

[3]Snow, Theodore P., and Benjamin J. McCall. "Diffuse Atomic and Molecular Clouds." Annual Review of Astronomy and Astrophysics 44 (2006): 367-414. Print.

[4]Wang, S., et all. ARCES: an echelle spectrograph for the Astrophysical Research Consortium (ARC) 3.5 m telescope. Proc. SPIE 4841 (2003): 1145.

Conclusion For this project, we report a non-detection of the doublet previously attributed to buckminsterfullerene cation.

The goals of future research will be: 1.! To continue performing correction for water vapor in the near infrared

region 2.! To use corrected spectra to identify any DIBs that may or may not be

related to PAHs

(cm)

In order to remove the water vapor absorption lines with minimal impact on underlying DIBS, the task ‘telluric’ was utilized in the Image Reduction and Analysis Facility (IRAF) (Fig. 4).

The task telluric contains two free parameters:

•! Scale: what multiplicative factor is used to adjust the sample spectra

•! Shift: the Doppler velocity used in order to match underlying stellar features

Telluric Correction

Figure 1: Absorption feature from PAH hypothesis

In the near infrared region (7000 – 10000 Å), potential DIB absorptions are obscured by strong water vapor absorption and molecular oxygen from the Earth’s atmosphere. If the telluric absorption can be removed, than by using a comparison sightline (which contains no DIBs) features may be able to be identified. The purpose of this study was

to correct for the strong water vapor

absorption in the near infrared in order to

identify absorption peaks for

buckminsterfullerene and other PAHs in

the interstellar medium.

Figure 2: Water vapor absorption in the near infrared

Figure 3: Region proposed to contain buckminsterfullerene (9577 Å & 9632 Å)

Figure 4: Removing atmospheric water vapor absorption lines

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