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Giovanni Covone » 8.Molecular clouds

History of observations of dark clouds

“Here is truly a hole in the heavens!” (W. Herschel)

Historical debate: truly voids (as theorized by Herschel) or dark regions observed projected upon a dense stellar background?

Barnard (1919) published a systematic photographic survey of the “Dark Markings of the Sky”.
The dark areas were “obscuring bodies nearer to us than the distant stars.”

In 1940, Bok recognized their association with star formation.

Second half of XX century: relationship between dark clouds and the formation of stars and planetary systems could be firmly established.

Weinreb et al. (1963): discovery of molecules in the interstellar space.

Molecular clouds

Terminology

Molecular Clouds (MC) appear dark because of the presence of dust grains absorbing optical stellar light.
High visual extinctions: $A_V > 1 \, {\rm mag}$ .

Densest and coldest regions of the ISM.

Cold dark clouds are sites of formation of low-mass stars (either in isolation or in small compact groups).
Gas temperatures: about 10 K to 20 K.

Diffuse clouds in the Milky Way contain molecular material but are not forming star.
Not optically opaque: $A_V < 1 {\rm mag}$ .

Massive giant molecular clouds form rich stellar clusters and contain embedded massive stars.
Gas temperatures: larger than 20 K.

Composition

Chemical and physical conditions of the interior of a molecular cloud are very different from those of the surrounding low-density ISM.

Local values density and temperature allow formation of molecules.

In the outer regions: hydrogen is neutral.
Inner regions: dust blocks large fraction of UV radiation; MC is darker and colder.

Forms of gaseous carbon: C+ (on the outside); neutral C (C0); the molecule carbon monoxide (CO).

At great depths within the cloud, other molecules can be seen from their microwave transitions, and more than 150 chemical species have been identified within the constituent gas.

“Exotic” chemistry because of the low densities and temperatures.

Example: the interstellar molecule HNC (hydroisocyanic acid) and its isomer HCN (hydrocyanic acid).

Properties of molecular clouds

Table: properties of molecular clouds, clumps and cores.

List of molecules in MCs

List of molecules in the ISM. Data taken from the Cologne Database for Molecular Spectroscopy. Adapted from N. Wehres (2011).

Density profiles

How to measure density distribution inside a dense molecular cloud.

NIR extinction from HK colors of background stars:

$A_V \, = \, 15.87 \, E(H-K) \, .$
The color excess:

$E (H-K) \equiv \, (H-K)_{\rm obs} \, - \, (H-K)_{\rm intrinsic} \, .$

The extinction is the converted to the column density by assuming that the gas/dust ratio is constant:

$N (H + H_2) \, = \, 2 \times 10^{21} {\rm cm}^{-2} \, {\rm mag}^{-1} \, A_V \, .$

The visual extinction through the cloud as a function of radius. Credit: adapted from Alves et al. (2001).

Bok globule B68

Central area of the Bok globule Barnard 68, in the optical and NIR. Credit: ESO.

Mass function of MCs

Importance of the mass function of MCs

How to measure the mass function.

From large-scale CO observations:

$\frac{dN}{d m} \propto M^{- \alpha}$
where $\alpha = 1.4 to 1.8$

From millimeter/submillimeter dust continuum peaks: $\alpha= 2.0 - 2.5$ , for masses larger than the solar mass.

Comparison with the Salpeter stellar Initial Mass Function (IMF) of stars.

Mass function of MCs compared with IMF. Credit: Alves et al. (2008).

The Circinus Molecular Cloud Complex

Star Formation in the Circinus Molecular Cloud Complex, observed by the Wide-Field Infrared Survey Explorer. Credit: NASA/JPL-Caltech/WISE Team.

MC distribution in spiral galaxies

Distribution of Molecular Clouds in two spiral galaxies, data from the BIMA Survey of Nearby Galaxies (SONG). Credit: Helfer et al. (2001).

Distribution of MCs and star-formation

Comparing distribution of neutral hydrogen, molecular gas with the distribution of the star-formation activity.

Result: star formation occurs mainly within the optical radius.

Relation between the star formation rate surface density $\Sigma_{\rm SFR}$ and the surfage gas density $\Sigma_{\rm gas}$ : the Schmidt (1959) law.

Recent determination of the Schmidt Law:
$\Sigma_{\rm SFR } \, = \, 10^{-2.1 \pm 0.2} \, \times \, \Sigma_{H_2}^{1.0 \pm 0.2}$

The gas depletion time:
$T \, \equiv \, \frac{M_{\rm gas}}{{\rm SFR}} \,<br /> = \, \frac{\Sigma_{\rm gas}}{\Sigma_{\rm SFR }} \sim 2 \times 10^9 \, {\rm years}$

Distribution of HI gas (left), CO gas (middle) and star-formation activity in NGC 6946. Credit Bigiel et al. (2006).

Star-Formation Rate indicators

How is SFR measured?

By measuring the flux contained in the $H_{\alpha}$ emission line:
${\rm SFR} \, (M_{\odot}/{\rm yr}) \, = \, \frac{L (H_{\alpha})}{1.26 \times 10^{41} \, {\rm erg/s}}$

Effect of dust on UV/optical radiation.

Using the FIR radiation:
${\rm SFR} \, (M_{\odot}/{\rm yr}) \, = \, \frac{L (FIR)}{2.2 \times 10^{43} \, {\rm erg/s}}<br /> \, = \, \frac{L (FIR)}{5.8 \times 10^9 \, L_{\odot}}$

Star-formation rate surface density from combining FUV and 24 microns radiation.

$\Sigma_{\rm SFR} \, [M_{\odot} \, {\rm yr}^{-1} \, {\rm kpc}^{-2}]\, = \,<br /> 3.2 \times 10^{-3} \, I_{24} \, [{\rm MJy \, ster}^{-1}]<br /> \, + \, 8.1 \times 10^{-2} \, I_{\rm FUV} \, [{\rm MJy \, ster}^{-1}]$