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Astron. Astrophys. 326, 1001-1012 (1997)

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4. Comparison with standard PMS evolution

We now compare the main properties of accreting PMS stars to those derived from classical pre-main sequence evolution (Siess et al. 1996b). We note that effective temperatures are very similar in both cases, a general feature that results from the fully convective status of these young low mass stars. The nuclear energy production due to deuterium burning heats up

the central part of the star and provides higher central temperature and smaller outer radius [Eq. (21)]. The effective temperature being confined in a narrow range, luminosity ([FORMULA]) is lower than in the standard scheme.

Comparing the apparent ages of accreting and non accreting PMS stars is a non-trivial matter.
Consider the position in the HRD where the evolutionary track of an accreting star crosses the standard PMS track of a star of given mass. Both stars have thus the same effective temperature [FORMULA] and luminosity L and so the same outer radius R, i.e. 5,

[EQUATION]

where t is the age of the star. If both stars are completely convective, they are located on the same Hayashi line and they thus have same mass provided their chemical composition is the same. Then, Eqs. (21) and (25) lead to

[EQUATION]

Since [FORMULA], we have

[EQUATION]

From the previous paragraph, we know that

[EQUATION]

and, since the central temperature is a strictly increasing function of time (as well as the mass of our accreting star)

[EQUATION]

it results, from relation (27) under the conditions imposed by Eqs. (28) and (29), that

[EQUATION]

In other words, accretion delays the evolution of accreting stars; they appear younger than in the standard evolution. To illustrate this point we have estimated the time at which the accreting star reaches 0.9 [FORMULA] and 1.2 [FORMULA] (Table 1). Note however that the maximum age differences are rather modest ([FORMULA] %).


[TABLE]

Table 1. Ages estimation from the position in the HRD when the different accreting stars (R1, R2 and R3) reach [FORMULA] [FORMULA] and when they first reach their standard evolutionary track at 1.2 [FORMULA]. At that time, the luminosity is [FORMULA] and the effective temperature [FORMULA]. [FORMULA] is the age of the accreting star given by the model and [FORMULA] its value deduced from the HRD location; [FORMULA].


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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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