Star formation is known to begin in molecular clouds (e.g. Zuckerman & Palmer (1974); Burton (1976)), made of gas, mainly molecular hydrogen but also carbon dioxide, and dust. Dust is an important component, because it protects molecules from dissociation due to ultraviolet (UV) radiation, and at the same time offers a medium for molecules likeH2 to form. There are two types of molecular clouds: giant molecular clouds (GMC),
like Orion; and small molecular clouds (SMC), like Taurus, Auriga and Ophiuchus. They differ in mass, density, size and temperature in the Galaxy, as shown in Table 1.1:
SMC GMC Mass <104M ⊙ 104 −6 M⊙ Density 10−19/−20g cm−3 10−19/−20g cm−3 Size 10-50 pc ≈100 pc Temperature 10-20K 50-100 K
Table 1.1: Differences between small and giant molecular clouds. From Hartmann (2009) and references therin.
GMCs and SMCs do not have uniform density, but present smaller structures like cores and clumps, which have increasing density and temperature, and decreasing size and mass compared to the larger molecular cloud. It is in these smaller and denser regions, having diameters not larger than 10 pc and masses between 103-104M
⊙ (e.g. reviews
by Cernicharo (1991); Williams et al. (2000)), that the collapse begins, giving origin to the formation of stars. Low mass stars can form in both GMC and SMC, whereas most massive stars are more likely to be found in GMC.
Several models have been proposed as mechanism to trigger the initial process of star formation, like for example turbulence, collisions of molecular clouds (Scoville et al., 1986) or explosions of nearby supernovae. The latter, first proposed by Opik (1953) and further
1.1. Formation of Young Stellar Objects
developed by Elmegreen & Lada (1977), has been recently confirmed by some authors (Chiaki et al., 2012), while others did not find any evidence between supernova remnants and star forming regions (Desai et al., 2010). Concerning the formation of molecular clouds, among the mechanisms proposed there are gravitational instability in the galactic disc (e.g Elmegreen, 1979; Balbus, 1988); converging flows, valid for clouds up to 104M⊙
as described in Dobbs et al. (2014); and spiral shocks (Bonnell et al., 2013).
Independently of the initial causes, when the mass is greater than a critical value, called the Jeans Mass, the cloud begins to collapse. The Jeans Mass, MJ, is defined as follows: MJ = 1.6 s 10 T K 3 cm−3 n M⊙ (1.1)
where T is the temperature and n is the number density. MJ can be several order of magnitude smaller than the total cloud mass, implying that molecular clouds would not be observable so extensively, because they would collapse much earlier as soon as their mass exceeds MJ. For this reason there must be some mechanisms to counteract gravity, like thermal gas pressure , turbulence (Norman & Silk, 1980; Larson, 1981), magnetic fields (Chandrasekhar & Fermi, 1953; Spitzer, 1968; Mouschovias, 1976) and rotation (Field, 1978). Turbulence and magnetic fields seem to be the main cause in larger clouds, while thermal pressure would play a role in small cores (Larson, 2003).
According to Larson (1969), for a typical core in a SMC having T ≈ 10 K, ρ = 10−19g cm−3,M = 1 M
⊙and size comprised between 0.1 and 0.4 pc the process of collapse
can be divided into a number of steps, where the main ones are described below.
1. “Isothermal phase”: in the initial phase the cloud collapses in free fall, and while the density is below 10−13g cm−3 the gravitational energy liberated is free to escape.
The duration of this phase is defined by the free-fall time:
τf f = r
3π
32Gρ (1.2)
which is ≈ 105 yrs. During the contraction of the molecular cloud, the density increases first in the centre rather than in the outer regions.
2. “Adiabatic phase”: when the density rises above 10−13g cm−3 the medium becomes
they can contrast the collapse, and a core in quasi hydrostatic equilibrium forms: the protostar.
3. “Formation of the second core”: once the temperature is above 2000 K the molecular hydrogen begins to dissociate, pressure drops and the core collapses even further. Since the energy is used for the dissociation of molecules, this process happen nearly in isothermal conditions. When all molecules are dissociated, temperature and pres- sure rise again and stop the collapse of the core. Density is now≈10−2
g cm−3
, but the mass is still small,≈10−2M
⊙.
4. “Accretion phase”: The core of the protostar is now in hydrostatic equilibrium, while most material is still accreting onto its surface. The luminosity of the protostar is given by the conversion into radiation of the gravitational energy produced during the shock on its surface. This phase corresponds to Class 0, described in the next section.
The further evolution depends on the mass of the protostars, which can be divided into two groups: low mass protostars, having M ≤ 3M⊙; and high mass ones, having
M >3M⊙. More massive protostars begin to burn hydrogen while they are still accreting
matter. In low mass protostars, instead, the accretion stops before they reach the main sequence, where hydrogen is burnt, and the luminosity comes from the gravitational energy liberated during the contraction that is still ongoing. The duration of this process is set by the Kelvin-Helmoltz timescale:
τKH ≈ GM∗2
R∗L∗
(1.3)
Low mass stars in the pre-main sequence phase are called T Tauri stars, after their prototype T Tau in the constellation of Taurus, and they will be the main subject of this thesis, especially in Chapter 3 and 4. Chapter 5 will include, instead, also some more massive ones, called Herbig Ae/Be stars.