r_min

[Мария Демьяненко]

Hi everyone!

I am a bachelor student, I started working with Cloudy and PyCloudy and have a strange problem: What is the inner radius? (Is equal r_min in PyCloudy?) I read Hazy and examples, and different articles. As far as I understand, r_min = inner radius in Hazy and is equal inner radius in paper 'INFRARED SPECTROSCOPY OF ULTRACOMPACT H II REGIONS'. Part of this paper: 

"The young O and B stars responsible for ionizing UC H II
regions are so deeply embedded in molecular cloud cores
that all of their UV radiation is absorbed by dust and rera-
diated in the IR. Churchwell, WolÐre, & Wood (1990, here-
after CWW) and WolÐre & Churchwell (1994) have
modeled the 1 mm to 1 km Ñux distribution of G5.89[0.39
by using the spherical radiative transfer code of WolÐre &
Cassinelli (1986). The Mathis, Rumpl, & Nordsieck (1977)
grain-size distribution was used with 25 discrete grain sizes
in the range 0.005 to 0.25 km for both graphite and silicate
grains. For G5.89[0.39, they found that a dust shell of
outer radius of order 1 pc, inner radius approximately 0.03
pc, and constant density of 1.7 ] 10~19 gm cm~3 produced
a satisfactory Ðt to the observed Ñux distribution."

So, Do I understand correctly? Is r_min just distance between young star and dust in H II regions? Why can't PyCloudy have for example r_min=0.1 cm? How can PyCloudy give a picture for zero approximation without dust/ without dust and gas? (I talk about Example 4 
https://github.com/Morisset/pyCloudy/blob/master/pyCloudy/docs/Using_pyCloudy_4.pdf)

Thank you for help! It is really important for me :(

With best regards,
Masha, 4-year student 

[Christophe Morisset]

Hi Masha,

You are right, the radius in Cloudy and pyCloudy is the inner radius of the nebula, that is the distance from the ionizing source. It is in log of cm in pyCloudy, for example 16 (that correposnds to 1e16 cm).

Something went wrong with the text you copied: the sizes of the grains are in micrometers, not kilometers. And the flux is computed from 1mm to 1 micrometer, these are wavelengths. Here is the text with (I hope) the correct typographies for the units (text from https://iopscience.iop.org/article/10.1086/305727/fulltext/ ):

The young O and B stars responsible for ionizing UC H II regions are so deeply embedded in molecular cloud cores that all of their UV radiation is absorbed by dust and reradiated in the IR. Churchwell, Wolfire, & Wood (1990, hereafter CWW) and Wolfire & Churchwell (1994) have modeled the 1 mm to 1 m flux distribution of G5.89-0.39 by using the spherical radiative transfer code of Wolfire & Cassinelli (1986). The Mathis, Rumpl, & Nordsieck (1977) grain-size distribution was used with 25 discrete grain sizes in the range 0.005 to 0.25 m for both graphite and silicate grains. For G5.89-0.39, they found that a dust shell of outer radius of order 1 pc, inner radius approximately 0.03 pc, and constant density of 1.7 × 10^-19 gm cm^-3 produced a satisfactory fit to the observed flux distribution. A large dust-evacuated cavity of radius 0.03 pc was required to be consistent with the observed low-IR fluxes. Average dust temperatures ranged from 25 K at the outer radius to about 290 K near the inner radius, with most of the decrease in temperature occurring between 9 × 10¹⁶ and 2 × 10¹⁷ cm.

Hope it helps,

Cheers,

Christophe