The characteristics of the IR emission features in the spectra of Herbig Ae stars: Evidence for chemical evolution

Abstract

Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of intermediate mass and are known to have varying amounts of natal cloud material still present in their direct vicinity. We characterise the IR emission bands, due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and link observed variations to spatial aspects of the mid-IR emission. We analysed two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with Spitzer and derive profiles of the major PAH bands. The shape and the measured band characteristics show pronounced variations between the two Spitzer spectra. Those variations parallel those found between three ISO spectra of other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to those from a broad sample of sources. The Spitzer and ISO spectra exhibit characteristics commonly interpreted respectively as interstellar matter-like (ISM), non-ISM-like, or a combination of the two. We argue that the PAH emission detected from the sources exhibiting a combination of ISM-like and non-ISM-like characteristics indicates the presence of two dissimilar, spatially separated, PAH families. As the shape of the individual PAH band profiles reflects the composition of the PAH molecules involved, this demonstrates that PAHs in subsequent, evolutionary linked stages of star formation are different from those in the general ISM, implying active chemistry. None of the detected PAH emission can be associated with the (unresolved) disk and is thus associated with the circumstellar cloud. This implies that chemical changes may already occur in the natal cloud and not necessarily in the disk.

References in corpus (19)

• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• The Spitzer Space Telescope Mission
• The Infrared Spectrograph on the Spitzer Space Telescope
• A Hipparcos census of the nearby OB associations
• The rich 6 to 9 um spectrum of interstellar PAHs
• ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
• The SMART Data Analysis Package for the Infrared Spectrograph on the Spitzer Space Telescope
• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• The profiles of the 3 to 12 $\mu$m PAH features
• The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics
• c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H_2 gas-phase lines
• Mid-infrared spectra of PAH emission in Herbig AeBe stars
• Spatially extended PAHs in circumstellar disks around T Tauri and Herbig Ae stars
• Spitzer-IRS spectra of disks around T Tauri stars II. PAH emission features
• Resolving the disk rotation of HD 97048 and HD 100546 in the [O I] 6300A line: evidence for a giant planet orbiting HD 100546
• Correlation between grain growth and disk geometry in Herbig Ae/Be systems
• PAHs in Circumstellar Disks around Herbig Ae/Be stars
• Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
• The 10 $\mu$m infrared band of silicate dust: A laboratory study comparing the aerosol and KBr pellet techniques

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• A Spitzer survey of mid-infrared molecular emission from protoplanetary disks I: Detection rates
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• CASSIS: The Cornell Atlas of Spitzer/Infrared Spectrograph Sources. II. High-resolution observations
• Spitzer Space Telescope spectra of post-AGB stars in the Large Magellanic Cloud ---polycyclic aromatic hydrocarbons at low metallicities
• Physical Properties of Fullerene-containing Galactic Planetary Nebulae
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• Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
• Polycyclic aromatic hydrocarbon ionization as a tracer of gas flows through protoplanetary disk gaps
• The 15-20 um emission in the reflection nebula NGC2023
• A Spitzer/IRS Spectrum of the 2008 Luminous Transient in NGC 300: Connection to Proto-Planetary Nebulae
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
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• The PAH hypothesis after 25 years
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• c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines
• The Cosmic Crystallinity Conundrum: Clues from IRAS 17495-2534
• Mechanochemical synthesis of Aromatic Infrared Band carriers. The top-down chemistry of interstellar carbonaceous dust grain analogues
• Mid-IR observations of circumstellar disks -- Part III: A mixed sample of PMS stars and Vega-type objects
• Evolution of PAHs in protoplanetary disks
• The 6 $\mu$m Feature as A Tracer of Aliphatic Components of Interstellar Carbonaceous Grains
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