Supplementary Material
to:
An Introduction to Radio Astronomy
4th edition Cambridge University Press 2019
Last updated 10/08/2019
Chapter 13: Stars and Nebulae
Stars
An accessible introduction to the life cycles of stars in the Milky Way
can be found at https://academic.oup.com/astrogeo/article/57/3/3.31/1741908
An animated summary view of star and planet formation and aspects of stellar evolution can be found at: https://exoplanets.nasa.gov/resources/2206/life-and-death-of-a-planetary-system/
Radio telescopes with high angular resolution, high sensitivity, and high spectral resolution have opened the rich field of radio emission from stars and nebulae. Recent developments in the study of radio stars were discussed at the MIT-Haystack Observatory Workshop 2017 , “Radio Stars: from kHz to THz”
see https://www.haystack.mit.edu/workshop/Radio-Stars2017/. Abstracts, many of the presentations and a conference summary by L.D. Matthews are available on this site; the conference summary is also available at https://arxiv.org/abs/1807.09798.
The combination of sensitivity and angular resolution is now sufficient resolve the disk of a few stars, such as the red giant Betelgeuse, but at present the main contribution of radio astronomy is to study circumstellar envelopes of one sort or another and the outflows from Young Stellar Objects (YSOs).
Red Giants
Betelgeuse:
The angular diameter of Betelgeuse, the first to be resolved optically by
Michelson, is ~0.055 arcseconds (near infra-red photosphere). An ALMA image at
338 GHz (0.89 mm wavelength) is slightly larger and shows two large hotspots.
The image and links to the scientific paper can be found at https://www.eso.org/public/unitedkingdom/images/potw1726a/
In the cm wave radio band the size and appearance of Betelgeuse is frequency
dependent due to material being ejected from the star. The figure above is from e-MERLIN at 5.75 GHz. The emission is from the radio photosphere and its size is about
4.5 times that of the optical star. The contours show the surface fluctuations
(up to ~10% of the average intensity) after subtracting a smooth disk from an
image at higher resolution. The plume to the north may be associated with
mass loss as similar features are seen in optical/IR interferometry sensitive
to molecules and dust. (image credit
Anita Richards)
R Sculptor: An ALMA image of mass loss from R Sculptor can be found
at https://www.eso.org/public/images/potw1807a/.
For an overview of
radio interferometer measurements of mass loss from cool stars see the
review by Richards et al https://www.zenodo.org/record/1220787#.XBEKz2j7Q2xA.
Asymptotic Giant Branch (AGB)
An international team of astronomers observed the
nearby evolved star L2 Puppis using ALMA. Five billion years ago, L2 Pup was an
almost perfect twin of our present-day Sun, so these observations allow us to
see the distant future of our own solar system see : http://www.jodrellbank.manchester.ac.uk/news-and-events/nearby-ageing-star-provides-glimpse-into-the-future-of-our-own-solar-system.htm
The diagnostic roles of masers
in the life history of stars
Masers are powerful tools to study two crucial
periods in the life of a star:
-
During the formation process of a high mass star
where the most common species are H20, CH3OH and OH
(also detected SiO, NH3, H2CO,
CH3CHO)
-
During the late stages of stellar evolution
(probing the outer parts of the circumstellar envelope) in Asymptotic Giant
Branch (AGB) stars , Protoplanetary Nebulae (PPN) and Ref super Giants (RSG) where the
most common species are SiO, H2O
and OH
(Figure from https://imagine.gsfc.nasa.gov/science/objects/stars1.html and Sandra Etoka
University of Manchester JBCA; text courtesy Sandra Etoka)
Silicon Monoxide maser clouds in shells around evolved
stars
Silicon Monoxide (SiO) masers around
the Mira variable star TX Cam imaged with the VLBA; the size of the star is
indicated by the red circle (see http://www.jb.man.ac.uk/news/2000/TXCam/). The differences between the
images on the two dates show that the maser spots move; in fact
the shell pulsates in an out as is made clear in a movie covering three
complete stellar cycles. The movie can be downloaded as supplementary material
at the end of the paper by I. Godinakis, P.J. Diamond
and A.J. Kemball (2013) https://academic.oup.com/mnras/article/433/4/3133/1749718 which also described the
history of the TX Cam observational programme.
Circumstellar Disks
Circumstellar
disks, with masses in the range 0.01 to 0.1 solar and sizes of 100s of AU, form
around young stars. They comprise solid material from which planets can emerge
but the majority of the mass is in the form of gas including complex,
pre-biotic, organic molecules. This entire research field is undergoing a rapid
transformation with both infra-red and in particular mm/sub-mm ALMA observations
playing a part. The ALMA observations involve both continuum (of
thermally-emitting dust) imaging and molecular spectroscopy.
Soon
after coming into full operation ALMA made a striking image of the
protoplanetary dust disk around the star HL Tau. The multiple
rings and gaps are associated with proto-planets sweeping their orbits clear of
dust and gas. Credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF) See also: https://www.almaobservatory.org/en/press-release/revolutionary-alma-image-reveals-planetary-genesis/
The Disk Substructures at High Angular Resolution
Project (DSHARP)
see
https://almascience.eso.org/almadata/lp/DSHARP/
and
is the largest homogeneous continuum survey looking at dusty
disks. To quote from the main survey website above
“DSHARP
is a deep, high resolution (35 mas, or 5 au) survey of the 240 GHz (1.25 mm)
continuum and 12CO J=2-1
line emission from 20 nearby, bright, and large protoplanetary disks, designed
to assess the prevalence, forms, locations, sizes, and amplitudes of
small-scale substructures in the distributions of the disk material and how
they might be related to the planet formation process.”
ALMA has made many other images
of dusty disks around stars, for example:
In Ophiucus:
https://www.eso.org/public/images/potw1809a/
V1247 Orionis https://www.eso.org/public/images/potw1742a/
Formalhaut: https://www.eso.org/public/images/potw1721a/
The story of pre-biotic molecules in circumstellar disks
is in its very early phases
Evolved Stars: A planetary nebula
An image of the planetary nebula
NGC7027 at 5 GHz made with a combination of VLA and eMERLIN
data (image credit Abeer Almutairi and Albert Zijlstra). NGC 7027 is the brightest known planetary
nebula at radio frequencies. The nebula
was ejected by the central star at the end of its evolution, and is now ionized
by the remaining core of the star whose temperature is >150,000 K. The
ionized plasma shell has a density of ~105 cm-3 and
has a physical
temperature of ~10,000K. The ionized shell is surrounded by a neutral and
molecular envelope which does not show in the radio continuum.
(This is a duplicate of the image
in Supp.Mat Chapter 2).
Young Stellar Objects (YSOs)
A diagram
from this article at http://adsabs.harvard.edu/abs/2016A%26G....57c3.31A shows an
emerging jet from the massive young stellar object GL 2591. Left) a radio composite: the red contours show the
6.5 GHz emission imaged with e-MERLIN at 0.080″ resolution; the grey
contours show the 8 GHz emission imaged with the VLA at 0.4″ resolution; the blue
contours show the emission at 203 GHz
imaged with the Plateau de Bure interferometer at 0.4″ resolution. The mm-wave
emission mostly traces the dusty disc perpendicular to the radio jet as well as
hints of jet emission (courtesy M.Hoare). Right: an near
infra-red image taken with the Gemini telescope view of the same region with a
resolution of 0.4 arcsec. (credit Gemini
Observatory/AURA). For more detail and references consult the original article.
The study
of jets from YSOs is a growing research
area as the combination of radio sensitivity and resolution opens up the
observational window – see for example
the paper by Purser et al http://adsabs.harvard.edu/abs/2016MNRAS.460.1039P. Perhaps the most
spectacular example of jets in a YSO is described in the paper by
Carrasco-Gonzalez et al https://www.epj-conferences.org/articles/epjconf/pdf/2013/22/epjconf_rj2013_03003.pdf
Massive star-forming regions: methanol masers as pointers
Type-1
methanol masers are pumped by collisional excitation while Type-2 methanol
masers are pumped by infra-red radiation associated with region of massive
start formation. These masers can be observed at both 6.7 GHz and 12.2
GHz.
A short illustrated introduction to methanol maser studies is given in http://www.naic.edu/~astro/highlights/Methanol_MasersPR.html which introduces Arecibo Methanol Maser Galactic Plane Survey under the title “Maser emission probes formation of massive stars and Milky Way’s structure”. It should be noted, however, that many other surveys for methanol masers have been carried out over the past 25 years. The paper by Olech et al (2019) see https://arxiv.org/pdf/1904.02649.pdf discusses the behaviour of some methanol maser sources which exhibit periodic variability and provides a copious references to previous methanol maser studies.
Microquasars and XRBs
The term microquasar indicates that these objects exhibit many of
the properties of AGN and are interpreted within a picture of accretion onto a
black hole.
An overview
presentation intended for an undergraduate audience see Microqlec3.pdf. A general review of microquasars
by Shuang-Nan Zhang (Frontiers of Physics vol 8, p630, Dec
2013) can be found at https://arxiv.org/abs/1302.5485. Recent results on one object and extensive references to other work
related to X-Ray Binaries (XRBs) can be found in the paper “Resolved,
expanding jets in the Galactic black hole candidate XTE J1908+094” by Rushton
et al. (2017) MNRAS, 468, 2788 (also at https://arxiv.org/abs/1703.02110)
Perhaps the best known microquasar
is the object SS433 within the supernova remnant W50 – see main text Fig 13.14.
The
article at https://motherboard.vice.com/en_us/article/kbb5km/the-manatee-nebula-is-a-space-cow-born-of-a-dead-star is a general introduction and shows striking pictures of W50 and includes a
movie of the precessing jet in SS433. The series of images below are from the
Jodrell Bank MERLIN array from observations at 5 GHz taken on 7 Dec; 12 Dec; 22 Dec 1991 and 4 Jan 1992. The images on the left were made with natural
weighting while those on the right were made with uniform weighting (see
Section 10.8.1); the former gives a greater sensitivity to lower brightness
emission at the expense of lower resolution (image courtesy
R.E. Spencer and F.
Jowett).
For
recent results on SS433 see Blundell et
al. (2018) ApJ Letts, 867, 25 also in https://arxiv.org/abs/1811.00760.
Nebulae
HII regions
Discussed in main text Section 13. 10. Radio recombination lines provide information on the physical conditions, in particular temperature, in ionised HII regions. They also provide information on atomic abundances (see the RRL entry in Chapter 3 of Supp. Mat.) For an overview of RRL astrophysics see M.A. Gordon and R.L. Sorochenko “Radio Recombination Lines” Astrophysics and Space Science Library , pub. Springer (2007).
Molecular Clouds:
The coverage of Molecular Clouds in the text is limited. These clouds are cold (10s of K)
agglomerations (sizes 10-100 pc; masses ~104 to ~106
solar) of gas and dust in which the density of particles (mean ~100 cm-3)
can be thousands of times higher than in the ISM in general. Dust obscuration prevents observations in the
visible band. Much of the gas is in
molecular form and so the diagnostic information is obtained from observations
of quantized rotational molecular lines in the short cm- and mm/submm-wave
regimes. Particular emphasis is placed on carbon monoxide (CO) observations
(lowest frequency transition 115 GHz – See Chapter 3) since it is abundant and the
dominant species, molecular hydrogen (H2 ),
has no electric dipole moment hence is radio quiet. For recent review article see: “Molecular
Clouds in the Milky Way” by M. Heyer and T.M. Dane Ann. Rev. Astr. Astrophys, 53, 583 (2015). Understanding the physics of molecular clouds
is an ongoing research topic but it is clear that compact cores in these clouds
are where stars are born (see the review by R.C. Kennicut
and N.J. Evans (2012) Ann. Rev. Astr. Astrophys, 50, 531).
The
field is constantly changing as observational capabilities advance. An up to
date (as of 2017) illustrated summary “Multi-molecular views of a stellar nursery” based on
the results of the Outstanding Radio-Imaging of Orion B (ORION-B) project (go.nature.com/2rca5py) can be found at https://www.nature.com/articles/nature22499. ALMA is also able to study molecular clouds in
external galaxies. For an overview presentation including a comparison of
external molecular clouds with the
galactic Giant Molecular Cloud W49 see http://www.alma.inaf.it/images/Paladino_mm15.pdf
Supernova
Remnants
A working catalogue of supernova
remnants is maintained by D. Green at www.mrao.cam.ac.uk/surveys/snrs
The latest
version is described in https://arxiv.org/abs/1907.02638