THE ASTRONOMICAL JOURNAL      VOLUME 106, NUMBER 2, PAGE 656        AUGUST 1993

       DUST EMISSION FEATURES IN 3 MICRON SPECTRA OF HERBIG Ae/Be STARS

                                 T. Y. BROOKE

    MS 169-237, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena,
                               California, 91109
                    Electronic mail: tyb@scnb.jpl.nasa.gov

                                A. T. TOKUNAGA
 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu,
                                 Hawaii 96822
               Electronic mail: tokunaga@galileo.ifa.hawaii.edu

                                  S. E. STROM
   Five College Astronomy Department, University of Massachusetts, Amherst,
                              Massachusetts 01002
                Electronic mail: sstrom@donald.phast.umass.edu


                                   ABSTRACT

    Low and medium resolution spectra in the 3 micron region were obtained
    of 24 Herbig Ae/Be stars in a search for organic features from the dust
    around young stars.  The 3.29 micron emission feature from aromatic
    hydrocarbons was detected for the first time in three objects, Lk
    Halpha 25, XY Per, and AS 310.  The width of the feature on Lk Halpha
    25 and XY Per corresponds to the narrow profile characteristic of
    material close to other young stellar objects.  Two other stars, HD
    245185 and HK Ori may have weak features.  About 20% of the Herbig
    Ae/Be stars surveyed to date have firmly detected 3.29 micron features.
    Among these stars, the feature-to-continuum contrast varies over a wide
    range; emission equivalent widths vary by a factor ~20.  The available
    data indicate that the 3.29 micron feature is more extended around
    Herbig Ae/Be stars of earlier spectral type, possibly due to
    dehydrogenization or destruction of the aromatics near these stars.
    Interpretation of the spectra in the optically thin limit suggests that
    the total number of aromatics excited by the stars is also greater
    around the earlier-type objects.  The ratio of 3.29 micron feature
    flux to far-infrared flux, which is a measure of the abundance of
    aromatics relative to larger grains, shows less variation; however,
    there may be real abundance differences in some of the stars surveyed.
    If the small grains around Herbig Ae/Be stars have spectral properties
    like those seen in reflection nebulas, then they play a significant
    role in the total energy balance of these objects; they can radiate
    between 7% and 60% of the far-infrared luminosity.  No other Herbig
    Ae/Be stars observed to date have the strong 3.42 and 3.53 micron
    features seen in Elias 1 and HD 97048.