Figure 7 shows a near-IR image from 2MASS5 with contours of Spitzer and [C II] overlaid. There is a higher stellar density at the location of the [C II] emission, coinciding well in size, shape, and position. There is substantial crowding in the 2MASS images and most of the fainter stars are typically red. On the red optical plates from the Digital Sky Survey only a handful of stars are seen. The extended emission seems brighter in the K-band. Judging from a preliminary J − H vs. H − K diagram, limited by source-confusion and sensitivity, the central cluster has a higher fraction of near-IR excess sources compared to the surrounding field. A preliminary color-color diagram also shows that sources with very different extinction values in the central cluster are found, though the median value is about 10 mag of visual extinction of the 2MASS sources measured. Many of the faint sources detected by 2MASS have no reliable flux measurement, however, and to make a detailed study of this cluster we need higher spatial resolution and deeper JHK images.
Based on these 2MASS images, both Kronberger et al. (2006) and Kumar et al. (2006) classified the crowding of stars as “an associated, partly embedded cluster”. The latter give 9 cluster members within an effective radius of 0.53 pc, and a stellar mass of 17 M⊙. Optical spectroscopy (Cohen et al. 1989) identified a B3 III star as one cluster member, but the spectral type classification is very uncertain because it comes from a rough fit to Balmer line depths, and the luminosity class III is derived from the required flux and not from a proper spectral classification.
Sridharan et al. (2002) derived a flux of 25 mJy at 3.6 cm and 1.4 Jy at 1.1 mm for this star, which is resolved at both wavelengths, and they interpreted it as a 90 M⊙ clump comprising a high-mass protostellar object and an H II-region. Independently, the spectral type of the ionizing source can be estimated from the relation between the radio continuum flux density and the Lyman photon flux (Martin-Hernandez et al. 2005). A flux of ~13.7 mJy at 1.4 GHz has been measured (Setia Gunawan et al. 2003), which is consistent – as is the flux information at 3.6 cm – with an ionizing photon flux of F = 1.8 × 1045 ph s-1, i.e. a B1 ZAMS star according to Panagia (1973).
Low-angular resolution millimeter continuum observations (BLAST, Roy et al. 2011; SCUBA, Williams et al. 2005) indicate a dust temperature of 38 K and a lower limit of the total mass of about 40 M⊙. Williams et al. determined a B2 spectral type from a luminosity of 4000 L⊙ and dust modeling. Using data from recent Herschel imaging within the HOBYS program (Motte et al. 2010), we obtained a total mass of 80 M⊙ for the globule (size scale ≈ 0.5 pc).
Though the spectral classification of the internal B-star is fairly uncertain (see above), we can estimate a flux considering different spectral types. Far-ultraviolet fluxes for a number of early B-type stars, which include α Vir (B1IV), α Eri (B3V), and β Cen (B1III), were measured by Holberg et al. (1982) using the UV spectrometers onboard the Voyager interplanetary probes. The spectral types of those three stars bracket the one estimated for the star at the center of the cluster, and their FUV luminosities can thus be taken as upper or lower limits
to the UV radiation internally injected in the globule. We used the distances to these stars as determined by Hipparcos (van Leeuwen 2007) to obtain their luminosities in the 912 Å–1225 Å range. We obtained luminosities of 1.1 × 1048 ph/s (α Vir), 9.7 × 1046 ph/s (α Eri), and 3.0 × 1048 ph/s (β Cen). The corresponding volume-averaged values of χ, adopting a typical radius of 0.2 pc for the globule, are χ = 4000 (α Vir), χ = 360 (α Eri), and χ = 11 000 (β Cen). These values are higher, or at least comparable for the lower limit set by the B3V star α Eri, than the peak value of χ derived from the IRAS 60 and 100 micron fluxes, thus supporting an internal origin for the excitation of the PDR.
We here consider only O-stars because they dominate the UV emission over B-stars, with O6 stars dominating the overall emission output from the cluster. The number of O-stars in Cyg OB2 is very uncertain and estimates range between ~45 and ~100 (see Reipurth & Schneider 2008 for an overview). In our estimates, we assume that G0 = 106 at a distance of 0.1 pc from an O-star. The projected distance from the center of the OB association to the globule is 40 pc. Hence, 100 O-stars produce a flux of G0 ≃ 108/4002 = 625 at the position of the globule, corresponding to χ ≃ 370. We consider this a strict upper limit. For 50 O-stars, this reduces to G0 ≃ 313 or χ ≃ 183.
The flux from the IRAS 60 and 100 μm flux ratio is G0 = 660 or χ ≃ 390 (obtained with a method described in Nakagawa et al. 1998) in a large beam at the position of the globule, which agrees well with the values estimated above. We believe, however, that the contribution to the FUV flux from the Cyg OB2 association is significantly lower than the upper limit derived above due to the reasons detailed below.
Because the volume around the OB association is not empty, there is still extinction on the way to the globule, which then does not receive the full UV flux from the entire O-star population. Unfortunately, it is not possible to give a precise quantification of the effect of extinction, but a reduction of the estimate of the flux by a factor of a few is possible.
A basic problem in estimating the number of O stars is the assumption of a single age for the whole association. Knödlseder (2000) used 3 Myr and thus assumed that all stars are on the main sequence, with the brightest stars, therefore, being hottest. Hanson (2003) already showed that some of the stars are bright because they are (B-type) giants and supergiants, not because they are hot. There are currently around 70 spectroscopically confirmed O stars in the association, but more than half of them are O8-O9.
Although it is already difficult to specify a distance between the O-stars and the globule, the distance of 40 pc to the center of the association is a projected distance. Taking into account the depth along the line of sight can only increase the actual distance, and because the external flux decreases quadratically with the distance to the cluster, this in addition may significantly decrease the FUV flux received by the globule.
© ESO, 2012