LAS CUATRO P’S

1.6.1.1 Mass-loss, stellar winds, and massive star evolution

As explored earlier throughout Section1.2, there currently exist substantial uncertainties in the mass-loss of massive stars. This fundamental process, responsible for the recycling of energy and material back into the ISM is in doubt. The mass-loss rates of luminous OB stars inferred from different observational diagnostics have been found to be discordant,

challenging the accepted model of mass-loss from stellar winds. Results from these diag- nostics, namely observations of the Hα line (λ =656nm), free-free emission in the sub-mm and radio, and those from ultraviolet (UV) resonance lines, have been found to differ on the order of magnitude scale (Prinja, Massa, and Searle, 2005; Fullerton, Massa, and Prinja,

2006; Puls et al.,2006). This discrepancy has far reaching consequences affecting the evo- lution and eventual fate of massive stars, the chemical enrichment of the local environment and that of the Galaxy, and the amount of material and energy available for the future generations of stars.

Massive stars have a huge influence in many areas of astrophysics. In the injection of energy, momentum and material back into the ISM, they drive numerous physical processes including turbulence, ionisation, heating and chemical mixing, all of which play a large role in planet formation and eventually the building blocks of life. Massive stars generate a huge amount of UV radiation, are the sites of cosmic ray acceleration and severely shape the environment in which they reside. Whilst creating kilo parsec scale super bubbles and wind-wind collision regions, they are also of great significance regarding the evolution of the universe as contributors to the epoch of re-ionisation. Their importance outlines the need for an accurate understanding of their fundamental parameters such as the extent of their mass-loss via clumped and/or porous radiation driven winds.

Mass-loss is a fundamental parameter that is key to the accuracy and success of stellar evolution models. The amount of mass shed by a massive star per unit time dictates its time spent on a particular evolutionary sequence and furthermore, promotes the existence of Wolf-Rayet (WR) stars, neutron stars, supernovae and black-holes by driving stellar evolution (see Section 1.1.3). OB stars have been found to lose mass via their radiation driven stellar winds (see Section1.2.1for further details). The electron-ion interactions that occur within their ionised winds produce radiation at radio wavelengths through thermal free-free emission. As shown in Section 4.2.2, this radio emission can be used to infer the mass-loss rate. As a ρ2 process, it is very sensitive to any wind-structure (or clumps) that may be present within the wind. Radio inferred mass-loss rates in comparison to the Hα or UV line diagnostics, potentially offer the most straightforward and least model dependant determination. Since the radio emission arises at large stellar radii from the photosphere

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where the terminal velocity of the wind will have been reached, this technique requires no knowledge of the photospheric profile or the velocity law of the wind. Furthermore, the radio flux is not strongly dependant on the ionisation conditions of the wind but along with observations in other spectral regions (mm, near-IR, Hα), can be used to constrain the run of the clumping factor as a function of stellar radii (Puls et al. 2006; see also Chapter 4

for further details).

In utilising e-MERLIN’s enhanced sensitivity to survey the ∼ 0.5 deg2 area of the Cygnus OB2 association at both L- and C-band, COBRaS aims to detect the thermal free-free radio emission, from a large number of the estimated 2600 ± 400 OB stars within the association. This will allow for reliable mass-loss estimates for a huge number of massive stars across a variety of evolutionary states. A comparison with other datasets across multiple wavebands will allow for an investigation into the amount of clumping within OB star winds. Moreover, COBRaS will conduct a study of the radio region clumping factor as a function of spectral type (and therefore as a function of stellar luminosity, effective temperature, etc), allowing for powerful constraints to be made on physical models for the origins of clumping.

1.6.1.2 Binarity and the incidence of non-thermal radiation.

The detection of non-thermal radiation in a cluster such as Cyg OB2 is highly indicative of a massive star binary since its production in single massive stars has been shown to be unlikely (van Loo et al. 2008; see also Chapter5). In, for example, an O+O type binary, the respective stellar winds collide causing shocks, around which electrons are accelerated to relativistic velocities. These relativistic electrons emit synchrotron radiation that can be detected at radio wavelengths (Dougherty and Williams,2000). A detected source from a single-epoch radio observation that is shown to be non-thermal and prior knowledge (from existing catalogues) that the object is indeed a massive star, is all that is required to identify massive star binaries. With observations at both L- and C-band, COBRaS will accurately determine the spectral index of each source detected. Cross-correlation of the results with existing catalogues will infer a better determined binary frequency in Cyg

OB2, a hugely important parameter for both evolutionary population synthesis models and dynamical simulations of stellar clusters. This in turn will have a broad impact on our understanding of the evolution and survival of massive star clusters and the chemical evolution of the Galaxy. Furthermore, massive star binaries have become an explanation for the existence of observed ratios between various post-main sequence objects. A better estimate of the binary frequency in massive stellar clusters will undoubtedly highly benefit a wide range of astrophysical science.

The study of non-thermal emission from the colliding wind region of massive star binaries is currently limited to a handful of examples, with three of these systems residing in the Cyg OB2 association (Contreras et al.,1997; Rauw et al.,2002; Blomme et al.,2005; van Loo et al., 2008; Blomme et al., 2010). As such, the massive star binaries detected with COBRaS will allow for a statistical investigation into the dependance of the non-thermal emission upon the binary parameters. Moreover, in the case of short period binaries, the detectability of synchrotron emission is uncertain due to the considerable amount of free- free absorption within their stellar winds (see Dougherty and Williams 2000). Cyg OB2 #8A, a massive star binary with a period of just 21.908 ± 0.040 days (Blomme et al.,

2010) has been shown to display phase-locked variations in its radio flux. The authors Blomme et al. (2010) model its behaviour to find that the synchrotron emitting region is not completely hidden by the free-free absorption. They also indicate that the clumping or porosity in the stellar wind could explain their observed radio spectral index and hence that non-thermal emitters such as Cyg OB2 #8A could also be used to investigate small- scale wind structure. Whilst the detection of massive star binaries will give us a better estimate of the binary frequency, they will also allow for the study of the colliding wind region itself in relation to the binary parameters and indeed the Fermi mechanism thought responsible for accelerating electrons to relativistic speeds.

Non-thermal radio emission is not only produced in the colliding wind regions of massive stars, but also from a variety of other pre-main sequence (PMS) objects such as Young Stellar Objects (YSOs) and T Tauri stars. The radio emission is produced due to the large amount of magnetic activity present in these PMS objects. Using the pre-existing X-ray catalogues and the relation between the X-ray and Radio luminosities (Güdel et

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al., 2002), there appears a substantial number of sources within Cyg OB2 that should be detectable with the COBRaS observations. Furthermore, flaring events will only increase the detectability of these objects. T Tauri stars are divided depending on the equivalent width of their Hα line into Classical T Tauri stars (CTTS; Hα width > 10 Å) and Weak T Tauri Stars (WTTS; Hα < 10 Å). These two classes of object will be able to be inferred since WTTS are found to be non-thermal emitters and CTTS appear to have similar radio properties to the PMO Herbig Ae/B stars, which are predominantly thermal in nature. The recent Chandra X-ray survey of Wright et al. (2014b) also determined ∼ 8000 X-ray point sources within the central 0.5 deg2 of Cyg OB2 with many of them considered to be YSOs. In cross-correlating the COBRaS radio catalogue with their X-ray findings, one will be able to directly test the relationship between the radio and X-ray luminosities for a large variety of YSOs.