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2010/2011 PH700 Project
Prof Michael D. Smith
Centre for Astrophysics & Planetary Science
University of Kent
Radio emission from young stars: predictions for LoFAR
Radio emission is associated with young stars on various scales: from their surface
regions, from outflowing jets, and from gas they ionise through their ultraviolet
radiation. In this project, these phenomena will be reviewed to determine their regimes
of importance.
Continuum radio emission (as opposed to line emission) from stellar jets can be
generated by synchrotron radiation or thermal free-free emission. This project will first
investigate the known radio jets to determine the emission process, flux and plasma
properties which produce it.
The radio fluxes produced from known radio jets are observed at centimetre
wavelengths. The aim here is to predict the emission that should be produced at the
longer metre wavelength, as will soon be searched for by the LoFAR radio telescope.
Massive stars eventually emit ultraviolet light which generates surrounding HII regions.
For these massive stars, the stage in which stellar jets are detectable will be placed into
context by developing a numerical model which follows their early evolution. Learning outcomes: ability to conduct a literature review, extract information and
equations in order to make new predictions. Ability to numerically solve equations and
analyse with Matlab or IDL. Ability to understand diverse physical processes and radio
astronomy. Consolidate knowledge learnt in astrophysics/physics modules and develop
computing skills. Gain insight into how a serious research topic is undertaken.
1. Literature Review http://ukads.nottingham.ac.uk/ads_abstracts.html
2. List specific issues
3. Get data/images ?
4. Establish what we know about yso radio jets
Bonn talk: Jochen Eisloeffel [[email protected]]
MK:
. From looking at the papers, protostellar jets can give us clues about the
state/conditions of the young star, and the overall goal is twofold; to build a coherent
picture of protostellar radio jets and to link it to star formation. In order to do that, we
need to look at:
The physical properties of the jets (length, velocity, flux density, emission process,
structure, angle of jet and beam width)
This would imply that one step we need to take is to derive relationships between the
jet observations and stellar parameters. For example, the length of jet
could give us the amount of material available, and thus what fraction
of mass is the jet? This is the most ambiguous part; I am unsure of
what to put where - can the velocity of the jet give us the energy
output of the star (i.e. Kinetic Energy linking with thermal energy)? I
don't know.
What the jets do. Do they extract the necessary angular momentum and if so is that
extraction confined to the radio continuum of the jets?
What causes the jets? The core? Disk? Outflow material interacting with the interstellar
envelope? A mix?
The velocity problem - there is a digression between the outflow velocity (~500 km/s)
and relativistic velocity of synchrotron emission.
The necessity of the jets. Is it a simple phenomenon akin to solar flares? Or is it a
necessary process to form the star (this links with the angular momentum
postulate)?
The physical conditions of the star needed to generate the jets.
Can a star change such that the nature of the jet would change over time? That is, can
a star emitting thermal change to emit nonthermal?
What conditions of the star cause it to favour one process over the other?
Can there be a real mix of the processes?
Does a particular type of YSO (HH, T Tauri etc.) emit one type?
Is there a turning point in the stellar parameters where one radiative process would
shift to the other?
Here is also a wild idea: What if the jets happen because the star is not yet hot
enough to fuse? After all, 10^4 K is what is needed for Bremsstrahlung and 10^6 K is
needed for Hydrogen fusion; failed stars don't have jets because there is no infall of
material. This would imply accretion is necessary for jets. This brings us a dilemma:
when would a YSO start producing jets, and when would it stop? Is it solely due to
availability of accretion material or are there thermal (or other) constraints involved
too? If we think about it like that, there is potentially a "Jet Phase" of a star's life
RADIO EMISSION FROM YOUNG STARS IN THE CONTEXT OF STAR FORMATION
MICHAEL J.W. KING
CENTRE FOR ASTROPHYSICS AND PLANETARY SCIENCE, UNIVERSITY OF KENT
Radio emission has been observed from stellar jets produced by Young Stellar Objects
(YSOs). While there has been substantial data collected based on the methodology of
identifying and thus detecting these radio jets, their exact nature is unknown and its
context in terms of a star's life remains a mystery. Furthermore there is no coherent
picture of these radio emissions because while there have been sources identified as radio
jets, the methodology can give different mechanisms, sources, and fluxes that would still be
attributed to these jets.
The purpose of this project then is twofold: to build a coherent picture of YSO radio
emission and to put it in the context of stellar evolution during its pre-main sequence
phase. The reason for this twofold focus is while the focus of the research will be on the
radio jets, it is necessary to put things in the right perspective and it is within this
dynamism of star formation that we can get ideas of the conditions needed for radio
emission.
This means that the project is a mix of theory and observation; the theory, in
particular stellar evolution and emission mechanisms, gives us the framework and
understanding in order to properly interpret the archived data from observations. The
interpretation will be done through collating the archived data and computer modelling in
IDL.
Refining the questions.
Below are some abstracts etc.
Q: Is there significant radio emission early and late, but not in the middle?
Q: Does the data show any correlation with the one parameter: alpha, the sp. Index?
See the Table in:
Radio jets on subarcsecond scale – a review:
http://adsabs.harvard.edu/abs/1996ASPC...93....3A
Can we update this table?
Q: How much radio emission from a jet with a certain outflow rate?
Q: Can jets radiate at metre wavelengths, or are they always bound to be well into the
optically thick regime where I propto mu^2 ??
Do cores hold protostars already? Radio emission would mean; yes. (or CO outflow, or
deep mid-IR emission, or X-ray emission).
YSOs: centrimentre emission is usually free-free from wind, jet or accretion surface.
Optically thick: I prop to frequency^2
Synchrotron: R CrA IRS5 and Serpens triple radio source.
Spectral index: see M0iettinen et al - Se section 2
http://adsabs.harvard.edu/abs/2008A%26A...486..799M
Gyrosynchrotron: low-gamma. High-gamma
Stage:
Class0/1 – thermal free-free from accretion or jet.
Class II: free-free from a strong ionized wind
Free-free absorbtion of any non-thermal by surrounding dense ionized gas: free-free may
dominate?
Class III: gyr-sych – from stellar field.
Gibb A 1999
http://adsabs.harvard.edu/abs/1999MNRAS.304....1G
perhaps only young stellar objects in the earliest and latest phases of protostellar
evolution exhibit detectable radio continuum emission. The earliest emission may be
due to bremsstrahlung from an ionized jet, which declines as a result of a decreasing
accretion rate or outflow efficiency. The later emission is due to gyrosynchrotron
processes arising as a consequence of a magnetic field in the vicinity of the star-disc
interface. Alternatively, a combination of the source geometry and optical depth may be
responsible for such an apparent correlation. Further observations of a larger sample are
required to test whether this correlation is seen in a wider view of low-mass star
formation.
http://adsabs.harvard.edu/abs/1996ApJ...473.1051M
Introduction:
http://adsabs.harvard.edu/abs/2007A%26A...462..677S
http://dialnet.unirioja.es/servlet/tesis?codigo=21599
We performed sensitive observations at 6 cm wavelength of the HH 80-81
jet revealing that the emission in the jet lobes, at ~0.5 pc from the driving
source, is linearly polarized. The detection of linearly polarized emission
confirms its synchrotron nature, implying the presence of relativistic
electrons and a magnetic field associated with the radio jet. Following
procedures similar to those used for extragalactic radio jets, that are
characterized by strong synchrotron emission, we inferred the structure and
strength of the magnetic field in the HH 80-81 jet. Our discovery of
synchrotron radiation in a jet from a YSO represents an important step in
the unification of the collimated outflow phenomena observed in many
astrophysical contexts.
Radio jets on subarcsecond scale – a review:
http://adsabs.harvard.edu/abs/1996ASPC...93....3A
A model for the thermal radio continuum emission produced by a shock wave and its
application to the Herbig-Haro objects 1 and 2
http://adsabs.harvard.edu/abs/1987RMxAA..14..595C
http://adsabs.harvard.edu/abs/2009AJ....137.5080D
We present high angular resolution observations of water masers at 1.3 cm and radio
continuum emission at 1.3, 3.6, and 6 cm toward the Bok globule CB 54 using the Very
Large Array. At 1.3 cm, with subarcsecond angular resolution, we detect a radio
continuum compact source located to the southwest of the globule and spatially
coincident with a mid-infrared (mid-IR) embedded object (MIR-b). The spectral index
derived between 6 and 1.3 cm (α = 0.3 ± 0.4) is flat, consistent with optically thin freefree emission from ionized gas. We propose the shock-ionization scenario as a viable
mechanism for producing the radio continuum emission observed at cm frequencies.
http://adsabs.harvard.edu/abs/2008AJ....135.2370R
The triple radio source detected in association with the luminous infrared source IRAS
16547-4247 has previously been studied with high angular resolution and high
sensitivity with the Very Large Array at 3.6 cm wavelength. In this paper, we present
new 3.6 cm observations taken 2.68 years after the first epoch that allow a search for
variability and proper motions, as well as the detection of additional faint sources in the
region. We do not detect proper motions along the axis of the outflow in the outer lobes
of this source at a 4σ upper limit of ~160 km s-1. This suggests that these lobes are
probably working surfaces where the jet is interacting with a denser medium. However,
the brightest components of the lobes show evidence of precession, at a rate of 0fdg08
yr-1 clockwise in the plane of the sky. It may be possible to understand the distribution
of almost all the identified sources as the result of ejecta from a precessing jet. The core
of the thermal jet shows significant variations in flux density and morphology. We
compare this source with other jets in low- and high-mass young stars and suggest that
the latter can be understood as a scaled-up version of the former.
http://adsabs.harvard.edu/abs/2004ApJ...612L..69S
We calculate the centimeter wavelength free-free emission of the jets of young stellar
objects (YSOs) with the X-wind model enhanced by a variety of physical processes.
Using parameters characteristic of a Class I YSO with a mass-loss rate of ~10-6 Msolar yr1, we obtain a 3.6 cm map and a spectral index that compare well with high spatial
resolution observations of L1551 IRS 5. Models with lower mass-loss rates, appropriate
for Class II YSOs with revealed optical jets, produce radio jets that are too weak to be
detected at current sensitivity levels. In addition to demonstrating the consistency of the
density distribution of the X-wind model with observations, we are able to obtain
information on the processes that heat and ionize the inner jet, i.e., X-ray ionization and
shock heating and ionization.