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.   

 

 

Radio image of the protoplanetary disk around young star HL Tau.

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

https://www.almaobservatory.org/en/press-release/alma-campaign-provides-unprecedented-views-of-the-birth-of-planets/

 

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

 

NGC 7027.PNG

 

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