Stars Herbig

In heavily obscured regions with ongoing star formation one observes the so-called Herbig-Haro (HH) objects thin collimated jets of matter rapidly flowing (up to several hundred kilometers per second) out from young stellar objects. An example is shown in Fig. 2.11. These jets are mainly associated with Class 0 and I objects but sometimes are also observed for T Tauri stars. The outflows interact with... 

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). 

Figure 7.3 Sketch of a protoplanetary disk surrounding a solar-type star showing the regions in which different techniques probe grain and particle sizes. More luminous stars, like Herbig Ae-type stars, will stretch the scale by up to a factor of 10, while less luminous stars, like brown dwarfs, will shrink it by a comparable factor. See Table 8.1 for stellar parameters of young stars.

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

Herbig-Haro objects emission-line nebulae which are produced by shock waves in the supersonic outflow of material from young stars also referred to as Herbig-Haro nebulae. 

Andre P. (1997) The evolution of flows and protostars. In Herbig-Haro Flows and the Birth of Low Mass Stars (eds. B. Reipurth and C. Bertout). Kluwer, Dordrecht, pp. 483 -494. 

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). 

Simon T., Herbig G., and Boesgaard A. M. (1985) The evolution of chromospheric activity and the spin-down of solar-type stars. Astrophys. J. 293, 551-574. 

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. 

Most young stars are surroimded by CS disks consisting of gas and dust (Strom et al. 1993). The inner parts of CS disks are ionized and some part of the polarized radiation of the Herbig AeBe stars can arise due to Thomson scattering of photons by free electrons. 

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. 

In the case concerning this chapter, a science sky datacube is the input to FllnS a simulation of a circumstellar disk around a Herbig Ae star kindly generated by Dr. Catherine Walsh at Leiden Observatory (Walsh et al. 2014). 

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)... 

T Tauri star An unstable young variable star in its pre-main sequence phase (see Hertzsprung-Russell diagram). The instability, brought about by the beginning of nuclear fusion in the core of the star, causes pulsations and stellar winds, possibly with bipolar outflows. Groups of such stars, often associated with Herbig-Haro objects, are called T Tauri associations. 

The study of the chemical and physical processes occurring in T Tauri outflows is of great importance as these stars have been proposed as the energy source of Herbig-Haro objects (Schwartz 1978 Canto 1978) and turbulence in molecular clouds (Norman and Silk 1980, Franco 1983). In this work, the physical model of the outflow is taken from Hartmann, Edwards and Avrett (1982) in which the primary stellar wind (i.e. wind that has not interacted with its environment) is ionized and heated by Alfven waves in the star s convection zone to reach a terminal velocity (of about 230 kms ) and a maximum temperature (of 20,000 K, cf. Hartmann et al. model no. 2) at z = 3 to 5, where z is the radial ordinate in units of stellar radii (r t 2 x 10 cm). Thereafter the wind expands and cools radlatively and adiabatically. Other parameters for the model are the initial wind density at z = 1 (oq lO - cm ), the density at z = 5 (n < 5 X 10 to 10 cm ) and the stellar photospheric temperature 4000 E). The cooling rate of the wind is obviously dependent on the physical conditions within the ejecta and in any case is by no means certain. Hartmann et al. suggest a... 

Herbig, G. H. (1962). The properties of T-Tauri stars and related objects. Advances in Astronomy and Astrophysics, 1,47-103. 

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. 

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