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Observed stellar magnetic fields and some associated phenomena

JD L

01/03/09

Leverhulme Lectures on Stellar Magnetism


Sunspot magnetism


Hale's discovery opened study of sunspot fields
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01/03/09

Sunspots typically have fields of 1- 3 kG Sunspots tend to come in pairs of opposite polarity In one hemisphere, north polarity usually leads and south trails, while in opposite hemisphere polarities are reversed During (roughly) 11-yr sunspot cycle, spots are formed closer and closer to equator, finally die out When new cycle starts, leading north polarity has moved to the opposite hemisphere: real sunspot cycle is 22 yr Sunspots regions are sites of most intense solar activity ­ flares, filaments & prominences Solar activity generally follows number and size of sunspots
Leverhulme Lectures on Stellar Magnetism


Variations of sunspot latitude and number during sunspot cycle

01/03/09

Leverhulme Lectures on Stellar Magnetism


01/03/09

Leverhulme Lectures on Stellar Magnetism


Solar outer layers in visible, UV, and X-ray

01/03/09

Leverhulme Lectures on Stellar Magnetism


Solar outer layers in visible, UV, and X-ray




Chromosphere and corona, unlike denser photsphere, are heated and completely structured by surface magnetic field UV images show chromospheric gas at ~50,000 K

01/03/09

Leverhulme Lectures on Stellar Magnetism


Solar outer layers in visible, UV, and X-ray






X-rays show corona at T ~ 1,000,000 K Dark regions are low density, where gas escapes as solar wind Bright regions are gas trapped by magnetic field
Leverhulme Lectures on Stellar Magnetism

01/03/09


Detection of fields in other stars








For nearly 40 years, no further fields found. Why? Sunspot fields could be detected because ­ Sun is extremely bright: high dispersion possible ­ Sunspot spectral lines not broadened by rotation ­ Sunspot fields are quite strong Zeeman splitting in stars (typically 0.01 A per kG) can be masked by even small v sin i of a few km/s In 1940s, H W Babcock exploited circular polarisation of Zeeman effect, using polarisation analyser on high dispersion spectrograph at Mt Wilson 100-in telescope He discovered kG fields in class of main sequence stars of masses around 2 ­ 3 M0 called "peculiar A" stars
Leverhulme Lectures on Stellar Magnetism

01/03/09


Typical observed characteristics of magnetic "peculiar A" (Ap) stars








Mean longitudinal field varies periodically, almost always approximately sinusoidally Usually the observed field reverses sign The star also usually varies in light (+/- a few 0.01 mag), and spectral line shapes and strengths vary, with same period as The period may be anywhere from 0.5 d to many years, and is inversely correlated with v sin i (large v sin i <=> short period) Clearly variations are due to rotation, and show that the star is not axisymmetric
Leverhulme Lectures on Stellar Magnetism

01/03/09


Typical Ap star variations (HD 119419)








Top: variations of equivalent width of strong spectral lines of Fe II and Si II Middle: brightness variations of star in Johnson UBV colours Bottom: variations through 2.60 day period Note that all variations are periodic with the same period
Leverhulme Lectures on Stellar Magnetism

01/03/09


Basic model of magnetic Ap the oblique (dipole) rotator










Sinusoidal variations of (including sign reversal) suggest that we are seeing rotation of a roughly dipolar field, inclined to the rotation axis of the star As star rotates we usually see first one pole and then the other Spectrum variations suggest that chemical composition of atmosphere varies over the surface: the star has "abundance patches" Abundance patches are probable cause of light variations Question: why aren't these mixed horizontally??
Leverhulme Lectures on Stellar Magnetism

01/03/09


Oblique rotator






Arrows show field vectors of roughly axisymmetric field Coloured bands could show abundance variations over surface Star rotates about axis not aligned with dipole axis
Leverhulme Lectures on Stellar Magnetism

01/03/09


Can we explain the data with a simple model?






Suppose surface field is produced by central dipole Simplest dipole model requires us to specify inclination of rotation axis i, "obliquity" of field axis to rotation axis, and polar field strength of dipole. Observations only provide two constraints (say, largest and smallest values of ), so the model will only be unique if we can get i from P, v sin i and stellar radius If we can measure average magnitude of field on stellar surface ("mean field modulus") as well as longitudinal field component, we get two more constraints and can fit a 4-parameter model
Leverhulme Lectures on Stellar Magnetism

01/03/09


Zeeman splitting in HD 94660

01/03/09

Leverhulme Lectures on Stellar Magnetism


Measuring the mean field modulus






If star has small rotation velocity (less than few km/s) and a large field (some kG), the Zeeman splitting of (at least some) spectral lines may be measurable Example of effect in HD 94660 Splitting is proportional to total magnitude of field, so we obtain a surface-averaged value of This also usually varies roughly sinusoidally With both and we can fit i, , and field strength at two poles (e.g. with a decentred dipole, or dipole + quadrupole) without detailed computations of line profiles
Leverhulme Lectures on Stellar Magnetism

01/03/09


Fit to and in HD 187474


01/03/09

Fit of simple dipole plus quadrupole model (lines) to observed variations of (top left) and (lower right) (cf Landstreet & Mathys 2000, A&A 359, 213)

Leverhulme Lectures on Stellar Magnetism


Better Ap field models


Results of such measurements and modelling:
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Clean splitting of spectral lines shows that field is fairly homogeneous over surface, not spotty as on sun Typically larges value of || is about /3 or /4, showing that field is not uniform, but is not much more complicated topologically than a dipole Fits to variations of and are reasonably good Show that in general the two magnetic poles are not equally strong, which is probably connected with the (at first surprising) observation that variable spectral line strengths are not similar at the two extrema of even if these are equal and opposite. If polar fields are different, chemistry could be too.
Leverhulme Lectures on Stellar Magnetism

01/03/09


Related magnetic stars










Fields are found in a few Herbig AeBe stars (pre-main sequence or ZAMS stars) Fields also found in some really massive O and B stars, up to ~50 M0. Magnetic OB stars not always chemically peculiar. Sometimes they have trapped winds Fields are similar to ApBp fields: B >~ hundreds of G, periodically variable, geometrically simple In Herbig AeBe stars absence of chemical peculiarities due to youth (?); in massive stars due to effects of strong stellar winds Will see later that fields with similar structure but much larger field strength are also found in some white dwarfs
Leverhulme Lectures on Stellar Magnetism

01/03/09


Do we expect to find any fields in lowmass (solar-type) stars?








In Sun, magnetic fields lead to chromosphere and coronal heating and structure Ca II H and K emission and activity, and even "solar cycles" are seen in lowmass MS stars X-rays on Sun reveal corona; most nearby lowmass stars show strong X-ray emission ---> => magnetic fields are common in low-mass stars
Leverhulme Lectures on Stellar Magnetism

01/03/09


Field in classical T Tau star BP Tau






Field is detected in CTTS (low mass PMS star) BP Tau by excess broadening of Zeeman sensitive Ti I lines compared to insensitive CO lines Field seems to cover most of star (from depth of Zeeman line wings) with |B| ~ 2 kG Zeeman broadening visible because of IR wavelength
Leverhulme Lectures on Stellar Magnetism

01/03/09


Magnetic field in M4.5V flare star








Compare EV Lac to other inactive M dwarfs in IR Magnetic field is detected in M4.5V flare star via Zeeman broadening Field |B| is ~4 kG, present over ~60% of surface Notice unbroadened TiO lines, slightly broadened Ti I line
Leverhulme Lectures on Stellar Magnetism

01/03/09


Polarimetry of T Tau stars








Fields of T Tau stars are also detected by circular polarimetry Amplitude of signal is very small, about 0.2% polarisation Sign of polarisation reverses several times, as would be expected from several large starspots, and net field is only ~ tens of G, compared to |B| ~ thousands of G These symptoms show fields are more complex than those of Ap stars
Leverhulme Lectures on Stellar Magnetism

01/03/09