IAU GA XXIX – Programme of the meeting of the IAU Working Group on Ap & Related Stars

Dear colleagues,

Please find below a first version of the programme of the meeting of the IAU Working Group on Ap & Related Stars, which will take place in Honolulu next Tuesday. I think that it is very exciting and covering a lot of ground, as you can also judge from the abstracts of the proposed talks. I am looking forward to a lively session, where I hope to see many of you.

Gautier Mathys
Chair, WG on Ap & Related Stars

Meeting of the

Working Group on Ap & Related Stars

IAU XXIXth General Assembly

Tuesday, August 11

Room 311

14:00 Opening of the meeting and business session

14:30 Scientific session (part 1)

14:30 Charles Cowley:

The abundance pattern of heavy elements in Sirius:
Impact of modern observations (STIS) and improved atomic data

14:50 Saul Adelman:

An Overview of my Current Work on mCP Stars

15:10 Luis Balona:

Activity and accretion in Chemically Peculiar A stars

15:30 Coffee break

15:50 Scientific session (part 2)

15:50 Elizabeth Griffin:

Peculiar and “Normal” Stars – Maintaining a healthy skepticism

16:10 Victoria Antoci:

What asteroseismology can teach us about chemically peculiar Am stars

16:30 Hiromoto Shibahashi:

Asteroseismic view of internal rotation of stars

17:00 Oleg Kochukhov:

Magnetic fields of early-type stars: recent results and future prospects

17:20 Gautier Mathys:

HD 18078: A very long period Ap star with an unusual magnetic field structure

17:30 John Landstreet:

Studying Ap stars in clusters

17:50 Closing remarks


1. The abundance pattern of heavy elements in Sirius:
Impact of modern observations (STIS) and improved atomic data

by C. R. Cowley, T. R. Ayres, G. M. Wahlgren, and K. G. Carpenter

We determine abundances or upper limits for the 55 stable elements from copper to uranium for the A1 Vm star Sirius. The primary observational material consists of Hubble Space Telescope (HST) spectra taken with the Space Telescope Imaging Spectrograph (STIS) from the ASTRAL project (Ayres 2010, ApJS, 187, 149). We have also used archival material from COPERNICUS (retrieved from the MAST) and from HST/GHRS, as well as the ground-based Furenlid, Westin, and Kurucz Sirius Atlas (FWK). The GHRS observations were described by Wahlgren, et al. (1993, Bull. AAS, 25, 1321). We also used the monumental study of Sirius by Klaus Kohl (1964, Zs. f. Ap. 60, 115, 1964, see also 1964, Das Spektrum des Sirius, 3100 – 8863A, Kiel thesis). Abundance determinations are based on the photospheric model of Landstreet (2011, A&A;, 528, 132). The atomic data base is significantly improved since the pioneering work by Sadakane (1988, PASP, 100, 811; 1991, 103, 355). The basic source was VALD3 (http://vald.inasan.ru/~vald3/php/vald.php), supplemented for all species by the essential NIST bibliographic data base (http://physics.nist.gov/cgi-bin/ASBib1/TransProbBib.cgi). We determine abundances and upper limits by synthesizing short wavelength regions around strong lines.

Virtually all of the abundance/upper limit results show excesses over the solar composition of between 1 and 2 dex. This result is in general agreement with overall results for metallic line stars, though we have no information on possible severe depletions for most elements. We conclude that the mechanisms causing abundance anomalies in Sirius have not acted to produce the extreme excesses of 4 or more dex (Pt, Hg), or deficiencies (Zn) seen in many HgMn stars.

CRC thanks Stefano Bagnulo for the UVESPOP Sirius spectrum. Robert Kurucz was most helpful with older Sirius UV and visual spectra.

2. An Overview of my Current Work on mCP Stars

by S. J. Adelman

I report on the status of the ASTRA Spectrophotometer, discuss progress with the automated spectrum synthesis program STELLAR of Graham Hill and Austin Gulliver, comment on the uvby photometry of HR 465, and then extend the paper coauthored with Dr. Robert J. Dukes, Jr., which was presented at the IAU GA poster session during the first week by discussing six additional stars.

3. Activity and accretion in Chemically Peculiar A stars

by L. A. Balona

Recent observations have shown that A stars are more complex than previously thought. About 40 per cent of A stars show photometric light variations with a period distribution matching that of rotation, indicating the presence of star spots. Furthermore, about 2.5 per cent of A stars show one or more flares. In fact, the relative numbers of A stars which flare is about the same as F and G stars and not much smaller than K and M stars. These observations are unexpected and need to be considered in the context of Ap and Am stars. Kepler observations show that Am stars have spots and flares, just as in normal A stars. The magnetic field strengths in Am stars must therefore be similar to those in normal A stars. Moreover the incidence of pulsation among Am stars is no different than in normal A stars. These observations contradict predictions of diffusion theory which require that magnetic fields be absent in Am stars and pulsation limited to the cool edge of the instability strip. I suggest that accretion of

planetary material might explain the distinction between Am stars and normal A stars. In support of this, I present circumstantial evidence indicating that close-in planets are common among A stars. I propose that the magnetic fields in Am and normal A stars are probably strong but tangled, whereas the magnetic fields in Ap stars are strong and ordered.

4. Peculiar and “Normal” Stars – Maintaining a healthy skepticism

by E. R. Griffin

Researchers need to be cautious about the wide application of a “model” to explain (whatever) results from research. When are the crossing-points between model -> hypothesis -> theory? Can the exception be a mere glitch of nature, or does it undermine all that we ever thought we knew? The A-stars have many examples of such situations, and we try to understand how we got to the situation where assertive statements overdo the actual case and harm the science that they are supposed to assist.

5. What asteroseismology can teach us about chemically peculiar Am stars

by V. Antoci

A large fraction of A-type stars are chemically peculiar Am stars, showing photospheric overabundances in the elements Ba, Y, Sr and underabundances in Sc and Ca when compared to the Sun. The Am phenomenon is related to atomic diffusion, which can efficiently operate because of slow rotation. Interestingly a significant number of Am stars are also pulsating delta Scuti stars, which still represent a mysterious group, as the reason why a large fraction of Am stars pulsates is not well understood. It is believed that, due to settling of He, this element is not sufficiently abundant in the He II zone for the kappa mechanism to drive pulsations. In this talk I will review the latest results based on Kepler observations and argue that it is most likely the turbulent pressure in the H ionisation layer triggering delta Scuti pulsations and not the kappa mechanism in the He II ionisation layer.

6. Asteroseismic view of internal rotation of stars

by H. Shibahashi

The internal rotation of stars, which undoubtedly affects the evolution of all stars in various ways, is only poorly understood and remains one of the unsolved fundamental problems in stellar astrophysics. The development of asteroseismic investigation is changing the situation, making it possible to observe the previously invisible stellar internal rotation. I demonstrate two cases of A-F main-sequence stars.

I will also show the long cadence observation of the K2 mission of Kepler Space Telescope is still useful to perform asteroseismic studies of those stars.

7. Magnetic fields of early-type stars: recent results and future

by O. Kochukhov

In this brief presentation I present selected highlights from recent observational studies of surface magnetic fields in A, B and O-type stars. I comment on the results obtained in the context of both large-scale surveys and detailed studies of individual objects. Ideas for future research projects, which would benefit from the coordination and collaboration within the Ap-star community, are put forward.

8. HD 18078: A very long period Ap star with an unusual magnetic field structure

by G. Mathys and I. I. Romanyuk

The rotation period of HD 18078, P = (1358 +/- 15) d, was determined from analysis of measurements of its mean longitudinal magnetic field and of its mean magnetic field modulus. HD 18078 is only the sixth Ap star with a period longer than 1000 d for which an exact value of that period has been obtained. Its longitudinal field and its field modulus vary in phase quadrature, which is quite unusual, and indicative of a magnetic field structure strongly departing from symmetry about an axis passing through the stellar centre.

9. Studying Ap stars in clusters

by J. D. Landstreet

Ap stars in open clusters provide a sample of magnetic stars with fairly secure ages. A sample of such stars has already been used to demonstrate evolution during the main sequence of both magnetic field strength and mean abundances. We are now gathering more detailed data on stars in this sample to explore the evolution of field structure and abundance maps with stellar age.