Observations of magnetic DZ white dwarfs Mark Hollands, Boris
Gänsicke, Detlev Koester,
Stelios Pyrzas, JJ Hermes
[email protected] References [1] Hollands et al., MNRAS 467,
4970 (2017) [2] Hollands et al., MNRAS 450, 681 (2015) [3]
Brinkworth et al. ApJ, 773 47 (2013) [4] Wickramasinghe et al.
PASP, 112, 873 (2000)
Figure 3: Our model for low-field Zeeman splitting applied to the
5F→5D Fe multiplet. Here we detect fields down to 250 kG from 2.5
resolution SDSS spectra. Beyond 0.5 MG, the reddest spectral lines
begin to show poor fits as spin and orbital angular momenta start
to decouple. At this field strength other transitions, either still
in the low-field regime or the Paschen-Back regime, can be used
instead for more accurate results.
Small magnetic felds ( 1MG) For an atom in a small external field,
the coupling of spin and orbital angular momenta can result in
complex splitting patterns. We have implemented a simple model
which can be applied to any multiplet given a sufficiently small
field. We use this model to estimate field strengths for several
low-field WDs using lines of Fe and Ca. This is demonstrated for
several objects in Figure 3.
Search for rotation SDSSJ1536+4205 exhibits a near uniform 9.6 MG
field on its visible hemisphere, requiring a complex field
geometry. As magnetic WDs typically have hours to days rotation
periods3, we reobserved this star four times with the goal of
surveying its other hemisphere and constraining the rotation
period. No change was seen suggesting either (a) the field and
rotation axes are almost aligned, (b) we look directly down the
rotation axis (unlikely), or (c) the rotation-period is at least
several decades long. Our data and fits are shown figure 2.
Normal Zeeman effect ( 1 MG) Measuring surface magnetic fields of
white dwarfs (WDs) in the Paschen-Back regime is simple. Atomic
lines split into 3 components with energy spacing proportional to
the field strength. We find 20+ WDs with >1MG fields from Zeeman
split lines of Mg and Na. The field geometry can also be probed by
integrating the field over the stellar disk. We show an
offset-dipole model applied to one WD in figure 1.
Quadratic Zeeman effect ( 10 MG) Very large magnetic fields can
result in further Zeeman splitting and line shifts. This is seen
for some magnetic WDs with H and He lines4, but is also seen here
for WDs with metal lines. DZH models should therefore aim to
include these effects.
The magnetic felds of cool white dwarfs are challenging to probe
due to the absence of strong hydrogen/helium lines. Accreted metals
provide the necessary spectral features to reveal substantial white
dwarf magnetism. In our sample of 231 SDSS DZ white dwarfs1,2, we
fnd 32 are magnetic (DZH) with felds ranging from 250 kG to
~30 MG. This large dynamic range of felds results in spectra
exhibiting anomalous, normal and quadratic Zeeman effects, which
upcoming magnetic model atmospheres will need to reproduce to
properly characterise these stars.
Figure 1: Our offset-dipole model fitted to the Na-D line of a
magnetic DZ white dwarf. We use a Bayesian MCMC routine to optimise
all free parameters, while including realistic priors on the pole
field strength Bd, the pole-observer inclination i, and the dipole
offset along the z-axis, az. In this case we find the data are
consistent with a centred dipole viewed near-equatorially.
Figure 2: The Na-D triplet of the SDSS data is shown in the left
panel. Our GTC data, taken three years later, and each separated in
time by about two weeks are shown in the remaining panels. We
fitted all spectra simultaneously using our offset-dipole model,
allowing the angle between the field axis and line-of-sight to vary
in each case. Evidently no change is seen on timescales of weeks or
years. Under the assumption of an offset-dipole, we also find Bd =
18.37±0.09 MG, and az = −0.230±0.003. The field implied for the
opposite hemisphere of this WD reaches 40.3±0.7 MG with this field
structure.
σ– σ+ π0
Figure 4: For SDSSJ1536+4205, one Zeeman triplet shows quadratic
Zeeman splitting. The transition likely corresponds to either Ca ii
or Na i subject to bluewards or redwards quadratic shifts.
Figure 5: In SDSSJ1143+6615, the Mg (blue) and Na (red) π0 lines
are blue-shifted from their usual wavelengths, indicating a field
of several 10 MG. Many of the other features remain unidentified at
this time.
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