By Guest Author 03/12/2016


In this guest post series astronomer Dr Yaël Nazé details her experience traveling from Belgium to New Zealand for the International Astronomy Union Symposia – The Lives and Death-Throes of Massive Stars.

Every conference has his favourite themes, which are the “hottest” things of the day. For IAUS329, there were several ones, plus some soft controversies (we won’t fight with fists or swords, right?).

Let’s begin with multiplicity. One often imagine stars living alone, as our own sun, but actually, many stars have companions (remember Tatooine?). For massive stars, large observation campaigns have led to clear statistics which were presented at the conference: more than half of the stars have a close companion, and if you consider larger separations, all massive stars have companions, there is no such thing as a single massive star – on average, such a star has two companions!

So what? Well, living alone and in couple (or trio or quartet) aren’t the same thing, right? For example (to take only a few mentioned at the conference): the companion can provoke tides on its neighbour, leading to a pulsating surface and a mixed interior which modifies its fate; when the companion becomes big, it can engulf its neighbour (common envelope phase, or CE) and/or give material to it (it’s called RLOF), again leading to a change in fate, including the possible merging of the stellar pair! What’s really funny is that a few years ago, such consequences were never mentioned at meetings – people laughed at the one daring to mention multiple stars. Today, it’s the contrary!

The symposium therefore had many contributions showing these consequences, like how material is ejected in a CE phase or what are the properties of a merged star (and supernova progenitor?). In the end, there is a paradox: the best single star actually resides in a binary – indeed, an apparently single star has 50% chance to have had its evolution affected by binary interactions (e.g. merging) while components of wide and young binaries have not interacted yet and thus have evolved “as if being alone”…

Gravitational waves and stellar quakes

Of course, no way to forget about gravitational waves. Here, we did not discuss the detections themselves but rather what they revealed: pairs of black holes of 10-40 solar masses. Now, how do you get that? Problem is that with all these interactions going on, a star could get its outer layers taken away by a companion or be the result of stellar merging, and in any case be too small for making a black hole! But tides exerted by a companion or mass transfer late in the life of stars could on the contrary help making decent black holes. Certainly, this all business about black hole pairs is not over, it’s only the beginning!

Finally, there are also advances and challenges. On the observational side, pulsations (aka stellar quakes) were recorded for massive stars, and were used to constrain the properties of the stellar interiors as well as to explain a mysterious broadening of stellar lines known as macroturbulence (up to now, a parameter added to have a nice fit, but without any physical basis). Precise observations of supernovae have also enlightened the ubiquitous presence of circumstellar material, sometimes from eruptions taking place just before the explosion.

On the theoretical side, 3D has arrived, for both stellar interiors and stellar environments! It makes things more complex, but also creates nice movies

Exploring the universe in 3D. Credit :Yael Naze.
Exploring the universe in 3D. Credit :Yael Naze.

 

There is still lot to do, though, including for very basic properties. The mass lost by massive stars (either a continuous flow or short-lived eruptions) remains quite difficult to constrain, but it’s a parameter that theoreticians need to build their models!

Magnetism

Another domain which is flourishing right now is magnetism of massive stars. The first detection occurred in 1978 for the star sigma Ori E, but it wasn’t followed by other ones until a few years ago, when instruments improved. In the meantime, magnetic fields were blamed each time there was something nobody could understand – an ad-hoc hypothesis without formal basis! At IAUS329, results of large surveys were unveiled: about 5 to 10% of massive stars are magnetic, and actually quite strongly magnetic (two third of these have magnetic fields between 0.6 and 10kG, with an average of 3kG – in comparison, the earth field is less than 1G).

Some groups are peculiar: Of?p stars are all magnetic, while Be stars never are. The origin of these fields is still not ascertained and many things remain to be explained (why do some magnetic stars rotating faster emit lots of X-rays while others don’t?) but progress is being made, and discoveries aren’t finished yet. For example, the evolved massive stars were not well studied before, but now, a few magnetic detections have been obtained, and the discussions began about the field evolution: is magnetic flux conserved or does it decay? We’ll need more data to answer that. Consequences of such fields for the stellar winds (the continuous ejection of material by these stars) are also being calculated, finally.

Another piece of big news was the mention of an internal magnetic field, which would arise from a dynamo effect inside the core of massive stars. We were shown how this could launch pulsations inside the star, or how this field could interact with the external one previously mentioned. It’s certainly a domain that’ll grow rapidly, as analyses of pulsations from lower mass stars have shown that about half of stars have such an inner magnetic field!

Stellar debates

Of course, a meeting is also a place to submit provocative ideas: what if young massive stars were still embedded in their natal cocoon, which would prevent their X-ray emission to be seen? What if the best way to find red supergiant stars (which are evolved massive stars) was to look at catalogues of quasars (i.e. very bright galaxy cores)? But there are also controversies. A few years ago, it was announced that stars with more than 300 solar masses existed. That provokes a huge debate, which is not finished, as we could see this week: the initial team and some colleagues maintain their conclusions, and have even enlarged them to cases in other galaxies, while others point problem in data analysis (the pollution by close neighbours) which would reduce the actual masses to 200, or even
100, solar masses. Debate is certainly not closed yet!

Finally, there is Nathan Smith. On the conference website, he registered under the title of “his holiness” rather than the classical Dr or Prof. He announced just before the conference, and mentioned during it, that the data from the GAIA space satellite have drastically reduce the distance of HR Car, a LBV (which is a type of evolved massive star), which would provoke an upheaval of our understanding of these objects. But several people have recalled that these first GAIA data were preliminary and with large error bars – once considered, this implies that HR Car can stay where we thought it was in the first place. Closed case? Certainly not, we have to wait until next year and the second data release to have the final solution to this potential problem.

The same guy also boldly proposed that LBVs were distributed in a different way than usual massive stars, implying that the connection between the two is not that obvious though LBVs are supposed to be one stage of massive stars’ lives… He has one fervent supporter who defended him brilliantly at the conference, but it’s also true that the main contradictors (who answered to Nathan in literature – see arxiv website) were not present at IASU329, which limited the potential fights. As it was mentioned this week: “Nathan’s right. Something’s odd”, but it could be the contrary – or even none of that. We’ll know more next time.

Dr Yaël Nazé is a FNRS research associate at the University of Liège (Belgium) where she studies massive stars, be them alone or in couple, and their winds. Her interests also concern the history of astronomy, the cultural impact of astronomy, and scientific outreach.

Featured image: NASA