Star Formation, Part II

So last week we talked about how new stars are formed from collapsing clouds of gas and dust, and how these protostars make their way towards the Main Sequence. This week, we’ll go over some of the different kinds of Pre-Main Sequnce objects as well as other things related to early stellar evolution that we find interesting.

T Tauri stars and their higher-mass counterparts Herbig Ae/Be stars are temperamental things, perhaps somewhat like toddlers (or teenagers). These young stars are still accreting matter from a thick surrounding disk and spewing it out in high velocity jets. The rotating disk is formed through conservation of angular momentum, since the initial collapsing cloud had at least a little bit of rotation to it, and therefore the speed of rotation increases as the cloud collapses. The jets produce Herbig-Haro objects, which are luminous patches of stuff moving away from a protostar. Material ejected from the star collides with ISM and causes it to glow brightly.

HH objects in Carina Nebula

Two Herbig-Haro objects in the Carina Nebula (yep, one of last year’s DSOs). One can easily see the outward jets and the bow shocks generated as things collide. (Credit: Hubble Heritage)

We know that T Tauri stars are young stars because of the amount of lithium they still contain in their atmospheres, since lithium is burned up quickly and cannot exist in such large quantities in old stars. They’re also irregular variables, with random changes in luminosity over a matter of days (mood swings, perhaps?). In fact, they’re so irregular that unlike basically every other kind of varstar, it’s impossible to classify stars as T Tauris based on their light curves and astronomers have to resort to looking at their spectra instead. From spectra, we can also distinguish two classes of these stars. Classic T Tauri stars (cTTs) have large accretion disks and show strong emission lines, while weak T Tauri stars (wTTs) hardly have any disk.

T Tauri spectra often show something called a P Cygni profile, a blueshifted absorption line right before an emission line (typically of hydrogen-alpha), first discovered in Luminous Blue Variable (LBV) P Cygni. This is a sign of mass loss, since the absorption line shows that light from the star is being absorbed by gas in front of it, and its blueshift means that the gas producing it is moving towards us, and therefore away from the star itself. The broad emission peak results from the fact that the star is expelling matter in all directions, and so some of it appears blueshifted to us and some of it appears redshifted.

P Cygni profile

Credit: Wikipedia

FU Orionis stars, or FUors (yes, they really are called that), are T Tauri-like stars that undergo sudden increases in mass accretion. Matter from the inner disk falls onto the star, both causing the disk to shine so brightly that it outshines the star itself and creating extremely high speed winds. It is thought that all T Tauri stars go through several FU Orionis “temper tantrums” before settling down on the main sequence.

To put it bluntly, brown dwarfs are slightly pathetic “failed stars” with masses less than 0.072 Msun but greater than that of a gas giant planet such as Jupiter. They simply don’t have enough mass to ignite the hydrogen fusion reactions necessary to form a main sequence star. Brown dwarfs generate energy mostly through the Kelvin-Helmholtz mechanism of gravitational contraction or through the burning of elements such as lithium or deuterium. We shouldn’t make fun of them too much, though, since there are a huge number of brown dwarfs in our galaxy and they’re extremely hard to detect (although Spitzer has discovered quite a few of them through observations in the infrared), so you never know where one may be lurking…

On the other end of the star-mass spectrum, there are things called OB associations and superbubbles (sadly you cannot have the fun of popping them).  OB associations are associations of O and B-type (i.e. high mass) stars that have similar radial or kinematic motions, forming what’s called a kinematic group.  They are an association also because they generally have similar ages, forming from the same collapsing gas cloud.  This goes into the study of stellar kinematics, which is relatively broad and won’t be fully mentioned, but it should be appropriately fascinating for all you evolving stars, I mean, readers out there.  Superbubbles come from high stellar winds associated with OB associations.  The reason they form is that multiple winds and shock waves from supernovae can form bubbles in a sort of spherical shape as they spread out from the star, and then combine with other bubbles to form superbubbles.

Superbubble in LHa120-N44

A superbubble. Isn’t astronomy pretty? (Credit: European Southern Observatory)

Some more fascinating features to our lovely early stars are circumstellar disks and protoplanetary disks (propylds).  Very simply, the circumstellar disk forms around a protostar as it spins and accretes a disk of material.  A propyld is when that disk is, as the name indicates, possibly able to form a planet.  So yes, we came from a bunch of spinning, hot stuff in space. To quote Carl Sagan and others, “we are star stuff.”  We are literally star stuff, yes, be happy for our stellar-ness and the fact that everyone’s relative is the Sun.  Moving on from our little side rant of how awesome space is, these disks are major hints as to protostars, their development, and how binary stars or planets are formed.

Well, we’ve gotten this far.  Yes, we’ve discussed quite a lot about the variety of ways that a protostar forms, and what it’s influenced by.  So, we have reached the Zero-Age Main Sequence (ZAMS)!  This may sound odd, but it makes quite a bit of sense.  It refers to the line across the H-R diagram for masses when a star is formed, or when it is at age zero on the main sequence (see what we did there).  There is in fact an inverse relation between star formation and time it takes to form.  This can reveal the Initial Mass Function (IMF), which basically defines again star formation.  It shows that most stars form with lower masses due to fragmentation and other reasons that make massive star formation more difficult.

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TL;DR — T Tauri stars are young stars still in the process of accreting matter, which they may spew out in jets to form Herbig-Haro objects. They often have P Cygni profiles in their spectra, which show that they are ejecting matter. FUors are thought to be T Tauri stars that suddenly have matter dumped onto them from the disk and increase greatly in brightness. Brown dwarfs are “failed stars” without enough mass for the fusion of hydrogen to helium to take place. A group of massive stars may be part of an OB association, which may in turn be home to a superbubble. Circumstellar disks and protoplanetary disks are… well, exactly what they sound like. Once a protostar has stabilized, it reaches the Zero-Age Main Sequence on the H-R diagram. The Initial Mass Function illustrates the formation of stars of different masses.

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Sources and links for further reading:

T Tauri/Herbig-Haro:

P Cygni:

FU Orionis:

Brown dwarf:

Superbubbles/OB:

Propyld:

In addition to this list, see last week’s sources/links.

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