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strange young stars



PV Cephei in October 2008, Adam Block/Mount Lemmon SkyCenter/University of Arizona


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Young Stellar Objects

Young Stellar Objects (YSOs) are stars in the first phase of theirs lives, before they enter the main sequence of the Hertzsprung-Russell diagram and are fed by stably hydrogen fusion. YSOs are formed by contraction (and fragmentation) of molecular clouds. Contraction can be started by a variety of factors, such as general density fluctuations in the interstellar medium, radiation pressure of nearby stars, or shock waves of supernova events that lead to local compressions. The contraction of the molecular cloud is driven by gravity, the cloud actually collapses in free falling. The gravitational energy is released by radiation and in turn influences the collapse by its radiation pressure, which counteracts gravity. The dense center of the molecular cloud is the new protostar. A protostar emits light due to the heat released by the gravitational collapse. Its core temperature, however, is still too low to maintain nuclear fusion. In this protostar stage, the star is still growing by mass accretion from the surrounding molecular cloud, which lasts until either the entire cloud is incorporated or until the radiation pressure of the new star is powerful enough to blow off the remainders of the cloud.


Protostars, proto-stellar disks, jets and Herbig-Haro-Objects

Due to the preservation of angular momentum, the molecular cloud cannot simply collapse. Instead, a protostellar disk is formed around the protostar during contraction of the cloud. Along the axis of rotation, in-falling material has only little angular momentum and the in-fall proceeds relatively unhindered. Therefore, the molecular cloud becomes thinner along the axis of rotation and two cone-shaped voids are formed at the poles, which allows light from the star to escape and to illuminate these cones from the inside. Depending from the viewing angle, we see the molecular cloud illuminated by the young star as a bipolar nebular (viewed from the side), as a fan-shaped nebula (at a low angle from above the disk) or in a crescent or even ring shape with increasing viewing angle.


Material migrates within the protostellar disk towards the star by internal friction: the protostar accretes material.


credit: ESO/L.CalÁada/M.Kornmesser

This image shows an artistic view of the dusty protoplanetary disk around a massive young star.


The spinning-up of protostar and protostellar disk winds up magnetic vortices, leading to strong magnetic fields along the polar axis and  the formation of bipolar outflows or jets. These jets may hit the surrounding interstellar medium or the remainders of the collapsing molecular cloud, leading to strong shock fronts, so-called Herbig-Haro objects (HHs).


credit: wikipedia


Relatively bright Herbig-Haro objects (HH1 and HH2) can be found, for instance, in a molecular cloud south of the Key Hole Nebula NGC 1999 in Orion at the lower edge of this image:


 Close up by HST press release


Patrick Hartigan at Rice University in Houston succeeded in detecting the dynamics within the jets and shock fronts of several HHs using the Hubble Space Telescope. Movies of these dynamics are available on his web pages.

Bewegung des Jets von HH 1, Patrick Hartigan


Further on on the evolution/contraction of the protostar, the mechanism of transport of the released gravitational energy out of the core of the protostar switches from convection to radiation. This leads to more efficient cooling of the core, which is important in particular for the heavier protostars, as it allows contraction to move on more rapidly. During this process, the star is found in the Hertzsprung-Russell diagram above the main sequence, moving downward along the so-called Hayashi line. Finally the core of the new star becomes hot and dense enough to maintain stable hydrogen fusion: A new star is born.


Herbig Ae/Be, T Tauri and FU Ori stars

During this transition stage, the new stars have not yet reached a stable hydrostatic equilibrium. Instead, they further contract despite that hydrogen fusion may have started. The new stars have not yet reached the main sequence, but are placed still above it. They are still larger and therefore brighter than main sequence stars of same temperature (and hence same spectral class). During this transition stage, the new stars are quite variable and may also show strong bursts of brightness (flares). These bursts reflect the erratic infall of material from the accretion disk onto the star. By excitation of the thin outer atmosphere, the stars display emission lines during this phase of their lives.

Stars smaller than 2 solar masses are classified as T Tauri stars, after their prototype, while the heavier ones are called Herbig Ae/Be stars (e stands for emission lines). The stars are identified by several criteria: presence of emission lines (in particular the Balmer series of hydrogen), an infrared excess of their radiation due to the enveloping dust in the surrounding disk, and their situation in a star forming region. The latter is verified by the projected location (for instance in a dark cloud) and the presence of a reflection nebula associated with the star, which secures the location of the star within the molecular cloud. The sub-types of YSOs are distinguished by spectral classification and their mass (B and A for Herbig Ae/Be stars and F, G, K, or M for T Tauri types). The pre-main sequence phase is in both cases short as compared with the entire life span of the star and lasts 1 to 10 million years for the massive Herbig Ae/Be stars to 10 to 100 million years for the less massive T Tauri stars.

T Tauri stars have a further sub-group termed FU Ori stars (or short "Fuors"), which due to erratic accretion are subjected to flares and very pronounced bursts of brightness of up to 6 magnitudes. It is conceivable that FU Ori-like behavior represents a phase during the development of most T Tauri stars. The FU Ori type is hence not a type of stars of its own, but rather represents a temporary stage during the evolution of a T Tauri star. In addition, there is also the Exor type (after EX Lupi), showing flares at shorter time scales.


Evolution of Young Stellar Objects

The evolution of a YSO is classified into four subsequent phases, which correlate more or less with the increasing exposure of the star in its envelope and the resulting changes of its spectrum (this classification scheme is actually based on the spectral energy distribution (SED) of the YSO).

adapted from Andrea Isella


During the first phase, the molecular cloud collapses to a protostar, that remains completely buried in its surrounding envelope, eluding direct observation in visible light. Only the infrared radiation of the warm envelope can be observed. This stadium of gravitational collapse is going along with a tremendous increase of the rotational frequency of the star and the accretion disk (spin up). In the following, the star becomes exposed and the spectrum of the YSO shows the black body spectrum of the emerging star, superimposed with the still substantial infrared excess of the envelope at lower frequencies. These stages are the classical T Tauri star stage (CTTS), with an active disk, accompanied with formation of jets and Herbig-Haro objects, and the weak-line T Tauri star stage (WTTS), with a passive disk, the onset of nuclear fusion and fragmentation of the protoplanetary disk. During the final stage, the star moves onto the main sequence with continuous hydrogen burning, the protoplanetary disk fragments into a planetary system (carrying most of the angular momentum of the YSO), and the infrared excess in the spectrum vanishes.

Image of the dust disk around the young star IM Lupi (upper) and other YSOs (lower). Images are from  SPHERE, ESO's Exoplanet Research Instrument at VLT in Chile.


A dusty neighborhood

Pre-main sequence stars are hence surrounded by (dusty) accretion disks and the (dusty) remainders of the molecular cloud in which they were formed. The accretion (or protoplanetary) disks are the cradles of new planets around the young star. The disk may often completely hide the star or at least strongly attenuate its light, thereby preventing its direct observation. Along the polar axis with its cone-shaped voids, however, the light of the star can pass relatively unhindered. In the telescope, this light can often be seen as it is illuminating the surrounding molecular cloud where it is scattered by dust particles. The bipolar appearance of the reflection nebula is hence a direct consequence of the presence of an accretion disk, which cuts the escaping star light down to narrow cones of light.


Variable nebula and polarization

Many of these reflection nebula are highly variable, which may have several reason:


The star itself is highly variable (e.g. during FU Ori-like phases)


The dusty cocoon around the star casts erratic and temporally variable shadows on the surrounding molecular cloud. These shadows propagate (by their very nature) with the speed of light.


A very impressive example of such casting of shadows is displayed by Hubble's Variable Nebula, NGC 2261, in the constellation of Monoceros. The animated image below shows a unique series of images by Tom Polakis from Tempe/Arizona, who photographed NGC 2261 over several weeks. The animation clearly shows the propagation of light and shadows along the reflection nebula.

variability of Hubble's Variable Nebula NGC 2261, Tom Polakis


Other reflection nebula with highly variable brightness are Gyulbudaghian's Nebula around PV Cephei, McNeil's Nebula in the molecular cloud of M78 in Orion, and the Nebula around Z Canis Majoris. The reflection nebula of PV Cephei, that used to be an easy target a few years ago with medium-sized telescopes, had been very difficult even with my 22" Dob over several years, until it brightened again in 08/2013.

Variability of the RN of PV Cephei on DSS red plates (compare Adam Block's image at the very of 2008)


McNeil's Nebula around V1647 Ori was discovered in 2004 by amateur Jay McNeil photographically. By 2006 it had dimmed again beyond visual detection. In 2008, it brightened again and had stayed on a relatively high level since then (this being the state of things until 2011), being accessible for telescopes around 18 inches or up.

McNeil's Nebula in 2006 (lower panel) and 2011 (upper panel).

credit: ESO/T. A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF and Igor Chekalin


All three stars, Z CMa, V1647 Orionis, and PV Cephei, are FU Ori variables with flares (with PV Cephei with its flares on shorter time scales, last flares were at 2004 and 2013, being rather an EXor). Another nebula that had been known for its brightness fluctuations on historical time scales is Hind's Variable Nebula around T Tauri.

A further phenomenon is the polarization of scattered light. Polarization of light can be verified by the appropriate filters, similar as described for the protoplanetary nebula. These have, despite the name, nothing in common with protoplanetary disks, but are formed during the late phases of stellar evolution prior to  formation of a full planetary nebula. Similar polarization-dependent observations of YSOs are naturally restricted to the brightest objects.


Formation of planets around YSOs

In a 2014 press release, ALMA has resolved the protoplanetary disk around the T Tauri star HL Tauri at 1.2mm wavelength into dense rings and gaps. These gaps presumably report accretion of disk material by evolving planets within the disk.


HL Tauri is situated in the same molecular cloud as Sharpless 239, and in its direct vicinity several other YSOs can be found:

ESA/Hubble and NASA


What can be seen of YSOs?

Visual observation of YSOs is like entering new territory. They are just too exotic and furthermore mostly very dim. There are only few well-known and brighter objects. The most well-known representatives are certainly Hubble's Variable Nebula and NGC 1999, which are rewarding objects, displaying structure already in smaller amateur telescopes. Further objects within reach of medium-sized telescopes are Ced 62 (NGC 2163) and Parsamian 21. Most other objects require large telescopes and are even then difficult targets. In many cases, only a stellar object can be discerned, the pre-main sequence star itself. The surrounding reflection nebula, so they are observable, are often extremely faint or outshined by the star. Their sometimes bizarre structure, which is very prominent on the DSS images, can be seen visually only in few cases. While it is usually not a problem to distinguish between star and surrounding nebulosity, the further distinction between reflection nebula, jet, and Herbig-Haro object is often difficult or not unequivocal. In particular the HHs are (despite few exemptions) very small and extremely faint. Nevertheless, these are very interesting objects, and it is exciting to observe stars in these very early stages of their lives (which is also the stage of the development of planets). And due to their high intrinsic variability, you never know what to expect!


The Young Stellar Objects Observing Guide

This observing guide introduces more than fifty pre-main sequence stars with surrounding reflection nebula with DSS images, finder charts, and observing reports at the eyepiece of my 22" Dob.


Download the Young Stellar Objects Observing Guide

 (pdf-File 15 MByte)

last update 03/2013


Many thanks to Sakib Rasool (www.starsurfin.co.uk), who contributed significantly in stimulating my interest in this class of objects ! 



Go on to Observing Reports of Young Stellar Objects


DSS images copyright notice

The Digitized Sky Survey was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.


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