6 H$\alpha$ flux in LH 101

Absolute H$\alpha$ photometry observations (Caplan $\&$ Deharveng 1985) in an aperture of 4.9$^\prime$ give an H$\alpha$ flux of LH 101 equivalent to a number of Lyman continuum photons per second ($N^{\rm obs}_{\rm Lyc}$) of 112 10⁴⁸ photons/s (Devereux & Scowen 1994) and corresponding to $\sim$3 OV stars (Panagia 1973). The number of Lyman continuum photons derived from our quasi complete O star content of LH 101 ($N^{\rm P}_{\rm Lyc}$) (Table 5) in the same area (Fig. 3) is found to be larger ($N^{\rm P}_{\rm Lyc} = 420 \ 10^{48}$ ph/s ) corresponding to $\sim$ 10 OV.

As seen in Fig. 3, the HII region is composed of three distinct zones. Using a few IDS nebular spectra of bright regions of LH 101 obtained with the ESO 1.5-m telescope in 1984, and our monochromatic CCD H$\beta$ image (Fig. 3), we calibrated each zone in flux F(H$\beta$) (Table 5).

We notice that as well in each HII zone as in the whole HII region, the $N^{\rm P}_{\rm Lyc}$ derived from the O stars is greater than $N^{\rm obs}_{\rm Lyc}$ derived from the observed H$\alpha$ or H$\beta$ flux by a factor 3-4. This discrepancy could be both attributed to inhomogeneity in the gas that would let escape Lyman continuum photons and to dust obscuration sensitive to H$\alpha$ (Caldwell $\&$ Kutner 1996). The reality of this density inhomogeneity is a basic property of young HII regions. Here we notice that the stellar population is strongly different in the S-E active HII region populated by unevolved O stars, and in the N-W and towards LH 104 where stellar winds probably have dispersed the interstellar matter.

  

Table 5: Number of Lyman continuum photons $N^{\rm P}_{\rm Lyc}$ derived from different Zones

Notes to Table 5:

Column 1, 2: Zone numbering , F(H$\beta$) in 10$^{-10}\,$erg s^-1cm^-2 in an aperture of 30$^{\prime\prime}$.
Column 3: $N^{\rm obs}_{\rm Lyc}$ in 10$^{49}\,$photons s^-1 corrected for the interstellar extinction.
Column 4: Westerlund identification.
Column 5: Spectral type.
Column 6: $N^{\rm P}_{\rm Lyc}$ derived from the ionizing stars (Panagia 1973).
Column 7: Extinction C(H$\beta$) from H$\alpha$/H$\beta$.