Category Archives: Wolf-Rayet Stars

The importance of massive stars

The mass range of stars drawing their energy supply from nuclear fusion covers about three orders of magnitude. The least massive stars known have masses around 0.1 solar masses (M) and the most massive examples are around 100 M, although stars with masses of ~150M may also exist.

Massive stars are defined as those stars with masses higher than around 8 M. However, this lower limit is not completely fixed, as the definition of massive star actually comes from those stars that ignite helium and afterwards carbon in non-degenerated stellar cores (i.e., the hydrostatic equilibrium is reached because the inward gravitational force is balanced with the outward force due to the pressure gradient of the gas). Depending on the evolutionary scenario, this happens between 7 and 9 M.

Massive stars consume their fuel faster than low and intermediate mass stars: a solar-mass star has a life ~125 times longer than a 10 M star. Massive stars also are very luminous: a 100M star shines with a luminosity similar to ~1600 Suns. Hence, except for stars of transient brightness, like novae and supernovae, hot, massive stars are the most luminous stellar objects in the Universe.

Young, massive star clusters near the center of the Milky Way, at ~25,000 light-years from Earth: the Arches cluster (left) and the Quintuplet Cluster (right). Both pictures were taken using infrared filters by the NICMOS camera of the Hubble Space Telescope in September 1997. The galactic center stars are white, the red stars are enshrouded in dust or behind dust, and the blue stars are foreground stars between us and the Milky Way’s center. The clusters are hidden from direct view behind black dust clouds in the constellation Sagittarius. Credit: Don Figer (STScI) and NASA.

Massive stars are, however, extremely rare. Following the very famous results obtained by the Austrian-Australian-American astrophysicist Edwin Ernest Salpeter in 1955, the number of stars formed per unit mass interval is roughly proportional to M -2.35. Therefore we expect to find only very few massive stars in comparison with solar-type stars: for each 20M star in the Milky Way there are roughly a hundred thousand solar-type stars; for each 100M star there should be over a million solar-type stars.

However, despite their relative low number, massive stars have a fundamental influence over the interstellar medium and galactic evolution because they are the responsible of the ionization of the surrounding gas and they deposit mechanical energy first via strong stellar winds and later as supernovae, enriching the interstellar medium by returning unprocessed and nuclear processed material during their whole life. Massive stars therefore condition their environment and supply it with new material available for the birth of new generations of stars, being even the triggering mechanism of star formation. They also generate most of the ultraviolet ionization radiation in galaxies, and power the far-infrared luminosities through the heating of dust. The combined action of stellar winds and supernovae explosions in massive young stellar clusters leads to the formation of super-bubbles that may derive in galactic super-winds. Furthermore, massive stars are the progenitors of the most energetic phenomenon nowadays found, the gamma-ray bursts (GRBs), as they collapse as supernova explotions into black holes. Particularly, the interest in hot luminous stars has increased in the last decade because of the massive star formation at high redshift and the results of numerical simulations regarding the formation of the fi rsts stars at zero metallicity (Population III stars), that are thought to be very massive stars with masses around 100 100M.

The descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (105  to 106 solar luminosities, L) O stars are Wolf-Rayet stars, which have typical masses of 25 – 30 M for solar metallicity.

The Crescent Nebula

A very nice example of a nebula surrounding a Wolf-Rayet star is the so-called Crescent Nebula (NGC 6888, Caldwell 27, Sharpless 105). Located in the northern constellation of Gygnus, The Swan, it lies at around 5000 light years from us. The Crescent Nebula has been formed by the strong stellar winds of the Wolf-Rayet star WR 136 (HD 192163), which is located in the center of the nebula. This is an image of the Crescent Nebula I took in 2004 using the 2.5m Isaac Newton Telescope (INT) at the Roque de los Muchachos Observatory (La Palma, Spain) while I was still preparing my PhD Thesis at the Instituto de Astrofísica de Canarias (IAC, Tenerife, Spain) about the properties of dwarf galaxies hosting Wolf-Rayet stars. Actually, the image was taken during the twilight, when sky is still dark enough the get details in the narrow-band filters.

Crescent Nebula using narrow-band filters, by Angel R. Lopez-Sanchez

Image of the Crescent Nebula (NGC 6888) obtained by the author combining data using the broad-band optical B filter (blue) and the narrow-band optical filters [O III] (green) and Hα (red) obtained using the Wide Field Camera (WFC) attached at the 2.5m Isaac Newton Telescope (INT) at the Roque de los Muchachos Observatory (La Palma, Spain). The size of the image is around 22 x 22 arcminutes, just slightly smaller than the field of view of the full moon in the sky (30 arcminutes in diameter). Credit: Ángel R. López-Sánchez

The complex structure of the Crescent Nebula is a consequence of the interaction of the strong wind of the Wolf-Rayet star with material ejected by the star in an earlier phase, probably while it was a red supergiant. The actual loss-mass rate of the WR136 is around 0.00001 solar masses per year, which means the star losses the equivalent of the Sun’s mass every 10,000 years.

The image clearly shows ionized gas (nebular emission) with very different conditions: while red-color (Hα emission) is tracing the normal, emitting ionized gas, the green colour ([O III] emission) indicates regions with high excitation of the gas, meaning higher temperatures probably because of shocks. In just some few hundreds of years the star will explode as type-II supernova and destroy all the nebula, although it will create a new object: a supernova remnant.

What are Wolf-Rayet stars?

Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200,000 K) and very luminous (105  to 106 solar luminosities, L) O stars, with masses 25 – 30 solar masses (M) for solar metallicity. WR stars possess very strong stellar winds, which reach velocities up to 3,000 km/s. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV ranges. These strong winds are also attributed to atmospheres in expansion. Actually, these winds are so strong that they are peeling the star and converting it in a nude nucleus without envelope. Indeed, WR stars have ejected their unprocessed outer Hydrogen-rich layers. WR stars typically lose 10−5 M a year; in comparison the Sun only loses  10−14  M⊙  per year.

Hα image of the Population I Wolf-Rayet star WR 124 (WN8) showing a young circunstelar envelope that is ejected at velocities highest than 300 km/s. The chaotic and filamentary structure created forms the M 1-67 nebula. The star is located at about 4.6 kpc from the Sun. At the left, image obtained by the author using the IAC-80 telescope, combining filters Hα (red) Hα continuum (green) and [O III] (blue). The right Hα image was obtained by the Hubble Space Telescope WFPC2 (Grosdidier et al. 1998). Note that the large arcs of nebulosity extend around the central star yet with not overall global shell structure. Furthermore, numerous bright knots of emission occur in the inner part of the nebula, often surrounded by what appear to be their own local wind diffuse bubbles. The dashed square in the IAC-80 image indicates the size of the HST image.

This is Figure 2.1 in my PhD Thesis.

WR stars were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. They found that three bright galactic stars located at Cygnus region have, rather than absorptions lines, broad strong emission bands superposed to the typical continuum of hot stars. In 1930 C.S Beals correctly identified these features as emission lines produced by high ionized elements as helium, carbon, nitrogen and oxygen.  The intriguing spectral appearance of WR stars is due both their strong stellar winds and highly evolved surface chemical abundance. In 1938, WR stars were subdivided into WN (nitrogen-rich) and WC (carbon-rich) depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen , respectively. Not until the 1980s did it became clear that WR stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. 

The mass-loss occurs via a continuous stellar wind which accelerated from low velocities near the surface of the star to velocities that exceed the surface escape speed. Their spectra, originated over a range of radii with the optical continuum forming close the stellar core and the emission lines in the more external areas (even beyond 10 stellar radii), indicate that the WR stars are embedded in luminous and turbulent shells of ejecta owing outwards at speeds comparable to the expansion velocities of novae although, in the case of WR stars, the expanding shell is being constantly fed with material from the main body of the star.

WR stars are extremely rare, reflecting their short lifespan. Indeed, they live for only some few hundred of thousands years, and hence only few WR stars are known: about 500 in our Milky Way and 100 in the Large Magellanic Cloud. Indeed, because of their peculiarities (brightness and broad emission lines), WR stars can be detected in distant galaxies. A galaxy showing features of WR stars in its spectrum is known as a Wolf-Rayet galaxy.

I compiled the main characteristics of WR stars in Chapter 2 of my PhD Thesis. A recent review about the properties of WR stars was presented by Crowther (2007).

From “El Lobo Rayado” to “The Lined Wolf”

Ángel R. López-Sánchez and the 2dF instrument at the 3.9m Anglo-Australian Telescope. Credit: Ella Pellegrini (Daily Telegraph).

Ángel R. López-Sánchez and the 2dF instrument at the 3.9m Anglo-Australian Telescope. Credit: Ella Pellegrini (Daily Telegraph).

Welcome to my new blog!

My name is Ángel R. López-Sánchez. I’m a Spanish astrophysicist working at the Australian Astronomical Observatory (AAO) and in the Department of Astronomy, Astrophysics and Astrophotonics of the Macquarie University (Sydney, NSW, Australia). My research in Astrophysics is focused in the analysis of star formation phenomena in galaxies of the local Universe, especially in dwarf starbursts and spiral galaxies, but using a multiwavelength approach.

In 2003, while starting my PhD thesis at the Instituto de Astrofísica de Canarias (IAC, Canary Islands Institute for Astronomy, Spain) I decided to create a blog about Astronomy to share with Spanish speakers my interests about this fascinating science. That was the birth of my blog El Lobo Rayado. I chose this title because in that moment I was analyzing a very interesting class of starburst galaxies, the so-called Wolf-Rayet galaxies. A bad translation from English to Spanish of Wolf (which means Lobo) and Rayet (which does not have a translation into Spanish, but it sounds like Lined) seemed a very original title for a blog about Astronomy, furthermore considering that then I was spending a lot of time analyzing optical spectra of galaxies showing many emission lines. In 2003 blogs were not as common as they are today, and I can say that El Lobo Rayado was one of the very first (if not the first) Spanish blogs fully dedicated to Astronomy and Astrophysics written by a Spanish astrophysicist.

I got my PhD in Astrophysics in December 2006. I presented a detailed analysis of a sample of Wolf-Rayet galaxies, the majority of the optical and near-infrared observations were obtained by myself using the telescopes available at the Spanish astronomical observatories of El Roque de los Muchachos (La Palma), Izaña (Tenerife) and Calar Alto (Almeria).

In 2007 I moved to Australia to work at the CSIRO Astronomy and Space Science (then just known as Australia Telescope National Facility) as radio-astronomer. Actually, I’m a weird mix between an optical and a radio astronomer, although I’m also using data from other wavelenghts. Indeed, I’m combining ultraviolet, optical, infrared and radio data to characterise the physical and chemical properties of galaxies and get clues about their nature and evolution. Since January 2011 I’m working at the Australian Astronomical Observatory (AAO) and Macquarie University (MQ) in Sydney (NSW, Australia).

My passion for Astronomy actually started when I was a kid, in the mountain ranges near my natal city, Córdoba (Spain), when I became an active amateur astronomer. Since 1991 I belong to the Agrupación Astronómica de Córdoba (AAC, Córdoba Astronomical Association). Besides being now a professional astronomer, I still feel like an amateur astronomer and indeed I enjoy a lot observing the sky with my eyes, binoculars or small telescopes and taking astronomical pictures using my own equipment.

I consider that outreach and publicizing Astronomy to the general public is very important and I’m usually involved in these activities. That is the reason I created both El Lobo Rayado and The Lined Wolf.

However, The Lined Wolf is not just a translation of El Lobo Rayado. Actually, I’m NOT going to translate a single post from one blog to the other. They will be complementary tools: I’ll continue writing in Spanish in El Lobo Rayado, as I consider it is very important to reach non-English speakers: the majority of the astronomical information, including press releases and hot news, is in English, and hence non-English speakers can still found some extra information about the most recent news about Astronomy in El Lobo Rayado. On the other hand, in the last few years I’ve been thinking it is also important for me to create my own blog about Astronomy in English. However, in this case my idea is to publicize my own research and explain the scientific papers I’m publishing, together with some of the adventures which involves to be a professional astronomer (observations in remote telescopes, conferences, workshops…). That is the aim that the blog The Lined Wolf has.

That also means I have to start from the beginning. There are many posts I still have to write here, just considering I’ll need at least a post per paper published in a refereed journal, plus posts showing some of my beautiful multiwavelength images. Shall we begin?