S11 - Formation of massive stars Flashcards
Why do massive stars have no visible pre-main sequence phase?
Free-fall time, t ∝ M^-1/2 whereas the time for collapse from protostar to main sequence, t ∝ M^-2, so for massive stars, the time to start main sequence is significantly shorter than the free-fall time of the molecular cloud collapsing into the star. As a result, massive stars arrive on the main sequence while still embedded in their collapsing molecular clouds
What is a Strömgren sphere?
The region around a massive star where hydrogen of the surrounding molecular cloud is ionised (HII region). Its volume and radius are measures of the amount of ionised hydrogen around a star.
What is the relationship, if any, between the number of ionising photons emitted by a star and its spectral type?
The number of ionising photons emitted by a star increases rapidly with increasing spectral type (which increases with mass).
At what wavelengths do massive stars emit most of their radiation?
UV wavelengths
What properties of a star are required for it to be able to ionise surround gas?
M>10 solar masses and T>30,000K
What radiation does ionised gas of the HII region surrounding a massive star emit?
The ionised gas of the HII region surrounding a massive star emits radio continuum, free-free radiation (Bremsstrahlung). This radiation is bright and is not absorbed by dust so can propagate unimpeded throught the ISM.
Why does the ionisation front (Strömgren radius) of a massive star expand and at what speed?
The ionisation front acts as a sharp boundary between the HI (molecular hydrogen) and HII (ionised hydrogen) regions. There is a large pressure difference at the ionisation front as ionised hydrogen has twice as many particles (H+ and e- compared to H2)(higher number density) and the temperature in the HII region is 1000 times higher than outside it (10,000K compared to 10K). The ionisation front expands at a speed of 10km/s due to the overpressure.
How much of a massive stars life is spent embedded within a molecular cloud?
About 15%
What are infrared dark clouds?
Cold and dense clouds that exhibit significant mid-infrared opacity (dark at mid-infrared wavelengths). They are thought to be sites of massive star formation.
What are MYSOs?
Massive young stellar objects. Bright at mid- and near-IR wavelengths. L > 10^4 solar luminosity. Bipolar molecular outflow, indicating ongoing accretion. Radio quiet so no HII region as not yet started main sequence.
What are UCHII regions?
The precursor of HII regions. In the far-IR, they are the most luminous objects in the galaxy.
Why is it likely that very massive stars accrete via disks like low mass stars?
As spherically symmetrical accretion of massive young stars suggests that the upper limit for star mass is below what is observed (radiation pressure would be too great to allow accretion once a certain mass limit below what has been observed is reached), accretion through a disk is a more logical explanation for the existence of stars that have accreted very massive amounts of mass.
