Herbig Ae/Be stars

Figure 6.5 Comparison of the 10 pm Si-O stretching bands of a GEMS-rich IDP and astronomical silicates. (A) Chondritic IDP L2008V42A. Profile derived from transmittance spectrum. (B) Comet Halley (Campins Ryan 1989). (C) Comet Hale-Bopp (Hayward et al. 2000). (D) Late-stage Herbig Ae/Be star HD 163296 (Sitko et al. 1999). The structure at 9.5 qm in (B), (C), and (D) is due to telluric O3. Figure from Bradley et al. (1999).

Two infrared spectral features have been attributed to nano-diamonds at 3.43 and 3.53 pm. A search for these features in a sample of 60 Herbig Ae/Be stars has resulted in only two significant detections of such diamonds, in HD 97048 and Elias 3-1 (see Acke van den Ancker 2006). These two sources are in no real aspect different from the rest of the sample. Habart et al. (2004) show that the diamond emission originates from the inner (i.e. < 15 AU) region of the disk. Also, the interstellar 3.47 pm absorption feature is attributed to nano-diamonds (see Pirali et al. 2007). 

Herbig Ae/Be star intermediate-mass ( 1.5-6 M ) star with a circumstellar disk, typically younger than 5-7 Myr. 

Hill H. G. M., Grady C. A., Nuth J. A., Hallenbeck S. L., and Sitko M. L. (2001) Constraints on nebular dynamics and chemistry based on observations of annealed magnesium silicate grains in comets and disks surrounding Herbig Ae/Be stars. Proc. Natl. Acad. Sci. 98, 2182-2187. 

The 11.2 pm fine structure on the Si-O silicate feature has provided interesting insight into the relationship between cometary and interstellar materials, because IR observations of silicates in the diffuse interstellar medium and molecular clouds do not show the feature (Molster et al., 2002a,b). Searches for the 11.2 pm fine structure towards the Galactic center indicates that less than 0.5% of interstellar silicates are crystalline (Kemper and Tielens, 2003). The crystalline olivine feature is, however, seen in certain astronomical objects, stars surrounded with disks. It has been seen in Beta Pictoris (Knacke et al., 1993) and Herbig Ae/Be stars... 

Waelkens et al., 1996) massive pre-main sequence stars surrounded with disks of dust and gas. Herbig Ae/Be stars even show transient gas features in their spectra that have been interpreted as comets falling into the star (Beust et al., 1994). The presence of the olivine feature in comets and circumstellar disk systems and the lack of it in interstellar and molecular clouds, the parental materials for star and planetary formation, is somewhat of a conundrum. A common astronomical interpretation is that interstellar grains are amorphous silicates and when warmed in a circumstellar disk environment, they anneal to produce crystalline materials. The other possibility is that olivine in comets and disks condenses from vapor produced by evaporation of original interstellar materials. 

The 10 p.m feature of chondritic IDPs has been compared with the 10 p.m feature of astronomical silicates. No particular IDP IR class consistently matches the —10 p.m feature of solar system comets or silicate dust in the interstellar medium (Sandford and Walker, 1985). However, the —10 p.m features of CP IDPs composed mostly of GEMS and submicrometer enstatite and forsterite crystals generally resemble those of comets and late-stage Herbig Ae/Be stars in support of the hypothesis that some CP IDPs are of cometary origin (Figure 10). 

Once the core temperature is high enough (hydrogen and helium must be fully ionized) the opacity ( bf+Kes) fall sufficiently for a radiative core to form. At this point the star contracts at constant L towards the main-sequence, and the convective envelope shrinks. By this stage, the stars have mostly emerged from their dusty cocoon and become visible, low-mass stars as T Tauri stars and intermediate mass stars as Herbig Ae/Be stars. 

After about one million years (for solar-mass stars, this process is much faster for higher masses), the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed, although a circumstellar disk is still present. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars (Hillenbrand et al. 1992). Several million years after the primordial disk has almost disappeared. 

The science datacube selected for the next simulations corresponds to a proto-planetary disk surrounding a Herbig Ae star 10,000 K). As presented earlier in this chapter, Herbig Ae/Be stars are pre-main-sequence stars. The main difference with T Tauri stars is the mass, this being Af+ > Mq. Spectrally, their SED shows strong infrared radiation excess due to the presence of the drcumstellar accretion disk (Hillenbrand et al. 1992), this is, the thermal emission of circumstellar dust. 

L.A. HUlenbrand, S.E. Strom, FJ. Vrba, J. Keene, Herbig ae/be stars-intermediate-mass stars surrounded by massive circumstellar accretion disks. Astrophys. J. 397, 613-643 (1992)... 

The location of the proposed PMS sources in both these plots suggests that < 5% are possible Class I sources, are Herbig Ae/Be stars, > 25% are classical T Tauri stars, and the remsdniug < 65% are low-mass stars. Of the low-mass stars, possibly lO are very low-mass stars. These are prime candidates for farther investigation under the on-going search for the elusive brown dwarfr. 

Abstract. I present first results of an infrared photometric study of the young starburst cluster NGC 3603. The in ared color-magnitude and color-color diagrams reveal a predominantly early-type stellar population and about 20 stars with significant IR excess which could be Herbig Ae/Be stars. These stars are distributed randomly in the cluster. 

The c-c diagram reveals a concentration of stars at J — iT 0.07 and H - K f>i 0.03, which would correspond to early A ZAMS stars. This is apparent discrepancy with the result of MTT who find that most stars are B2 or earlier. A more accurate absolute calibration of our photometry may resolve this issue. Some 20 stars are located well to the right of the reddening line, indicating that they have significant IR excesses. The location of these stars suggest that they are Herbig Ae/Be stars (see Lada and Adams 1992). Analysis of the spatial distribution of these IR excess stars shows a rather random distribution. 

Fig. 1. 2.2 itm. polarization vector map of nebulosity surrounding the Herbig Ae/Be star LkHa 234. The outer contour is at 13.6 per square arcsec the inner contour, at 7.18 per square arcsec, marks the position of the star. Axis A, whidi bisects the polarization pattern to the southwest of the star, appears to be centered on a deeply embedded star (marked by the star symbol) that is not detected at 2.2 pm. Axis B is the axis of the polarization pattern to the northwest. This pattern appears centered on LkHa 234. 

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