Observational Evidence Linking Interstellar UV Absorption to PAH Molecules

Abstract

The 2175 Å UV extinction feature was discovered in the mid-1960s, yet its physical origin remains poorly understood. One suggestion is absorption by Polycyclic Aromatic Hydrocarbons (PAH) molecules, which is supported by theoretical molecular structure computations and by laboratory experiments. PAHs are positively detected by their 3.3, 6.2, 7.7 8.6, 11.3 & 12.7 $\mu$m IR emission bands, which are specified by their modes of vibration. A definitive empirical link between the 2175 Å UV extinction, and the PAH IR emission bands, however, is still missing. We present a new sample of hot stars that have both 2175 Å absorption and PAH IR emission. We find significant shifts of the central wavelength of the UV absorption feature, up to 2350 Å, but predominantly in stars that also have IR PAH emission. These UV shifts depend on stellar temperature in a fashion that is similar to the shifts of the 6.2 and 7.7$\mu$m PAH IR bands, namely the features are increasingly more red-shifted as the stellar temperature decreases, but only below $\sim 15$ kK. Above 15 kK both UV and IR features retain their nominal values. Moreover, we find a suggestive correlation between the UV and IR shifts. We hypothesize that these similar dependences of both the UV and IR features on stellar temperature hint to a common origin of the two in PAH molecules and may establish the missing link between the UV and IR observations. We further suggest that the shifts depend on molecular size, and that the critical temperature of $\sim 15$ kK above which no shifts are observed is related to the onset of UV driven hot-star winds and their associated shocks.

arXiv commentsubmitted, comments welcome

References in corpus (21)

• Interstellar Dust Grains
• Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars
• An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
• New Interstellar Dust Models Consistent with Extinction, Emission, and Abundance Constraints
• The rich 6 to 9 um spectrum of interstellar PAHs
• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
• The profiles of the 3 to 12 $\mu$m PAH features
• On-line database of the spectral properties of polycyclic aromatic hydrocarbons
• Ultraviolet Extinction Properties in the Milky Way
• The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics
• Do the Infrared Emission Features Need Ultraviolet Excitation? The PAH Model in UV-Poor Reflection Nebulae
• Spitzer's view on aromatic and aliphatic hydrocarbon emission in Herbig Ae stars
• Dust in the diffuse interstellar medium: Extinction, emission, linear and circular polarisation
• Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump
• An Analysis of the Shapes of Ultraviolet Extinction Curves. IV. Extinction without Standards
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
• Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?
• Carriers of the mid-IR emission bands in PNe reanalysed. Evidence of a link between circumstellar and interstellar aromatic dust
• Observations of the Near- to Mid-Infrared Unidentified Emission Bands in the Interstellar Medium of the Large Magellanic Cloud
• What can we learn about protoplanetary disks from analysis of mid-infrared carbonaceous dust emission?

Cited by in corpus (16)

• Carbonaceous dust grains seen in the first billion years of cosmic time
• Examining the class B-to-A shift of the 7.7 $\mu$m PAH band with the NASA Ames PAH IR Spectroscopic Database
• Adsorption of Organic Molecules on Onion-Like-Carbons: Insights on the Formation of Interstellar Hydrocarbons
• Relations between mid-IR dust emission and UV extinction
• Toward understanding physical origin of 2175Å extinction bump in interstellar medium
• Detection of the 2175Å UV Bump at z>7: Evidence for Rapid Dust Evolution in a Merging Reionisation-Era Galaxy
• Far-ultraviolet Dust Extinction and Molecular Hydrogen in the Diffuse Milky Way Interstellar Medium
• The 2175 Angstrom Interstellar Extinction Bump: Is the Wavelength Variable?
• Ultraviolet Spectropolarimetry with Polstar: Interstellar Medium Science
• Exploring extreme brightness variations in blue supergiant MACHO 80.7443.1718: Evidence for companion-driven enhanced mass loss
• The 2175 Å~ Bump Features in FeLoBAL Quasars: One Indicator of MW-like Dust in the Nuclear Region of Quasar
• CHESS: an Innovative Concept for High-Resolution, Far-UV Spectroscopy
• An Infrared Photometric and Spectroscopic Study of Post-AGB Stars
• Diffuse Interstellar bands and ultraviolet extinction bump: a Milky Way perspective on distant galaxies
• Space-filling efficiency and optical properties of hemoglycin
• Large Interstellar Polarisation Survey. III. Observational constraints on the structure of grains