21 research outputs found
Least squares deconvolution of the stellar intensity and polarization spectra
Least squares deconvolution (LSD) is a powerful method of extracting
high-precision average line profiles from the stellar intensity and
polarization spectra. Despite its common usage, the LSD method is poorly
documented and has never been tested using realistic synthetic spectra. In this
study we revisit the key assumptions of the LSD technique, clarify its
numerical implementation, discuss possible improvements and give
recommendations how to make LSD results understandable and reproducible. We
also address the problem of interpretation of the moments and shapes of the LSD
profiles in terms of physical parameters. We have developed an improved,
multiprofile version of LSD and have extended the deconvolution procedure to
linear polarization analysis taking into account anomalous Zeeman splitting of
spectral lines. This code is applied to the theoretical Stokes parameter
spectra. We test various methods of interpreting the mean profiles,
investigating how coarse approximations of the multiline technique translate
into errors of the derived parameters. We find that, generally, the Stokes
parameter LSD profiles do not behave as a real spectral line with respect to
the variation of magnetic field and elemental abundance. This problem is
especially prominent for the Stokes I variation with abundance and Stokes Q
variation with magnetic field. At the same time, the Stokes V LSD spectra
closely resemble profile of a properly chosen synthetic line for the magnetic
field strength up to 1 kG. We conclude that the usual method of interpreting
the LSD profiles by assuming that they are equivalent to a real spectral line
gives satisfactory results only in a limited parameter range and thus should be
applied with caution. A more trustworthy approach is to abandon the single-line
approximation of the average profiles and apply LSD consistently to
observations and synthetic spectra.Comment: Accepted for publication in Astronomy & Astrophysics; 15 pages, 12
figures; second version includes minor language correction
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
According to our current understanding, a subclass of the upper main sequence
chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic.
Nevertheless, chemical inhomogeneities were recently discovered on their
surfaces. At the same time, no global magnetic fields stronger than 1-100 G are
detected by modern studies. The goals of our study are to search for magnetic
field in the HgMn binary system 66 Eri and to investigate chemical spots on the
stellar surfaces of both components. Our analysis is based on high quality
spectropolarimetric time-series observations obtained during 10 consecutive
nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study line profile variability. Chemical spot geometry was
reconstructed using multi-line Doppler imaging. We report a non-detection of
magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field.
Circular polarization profiles also do not indicate any signatures of complex
surface magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the HgMn
component we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these elements
reconstructed with the Doppler imaging technique, show relative underabundance
on the hemisphere facing the secondary component. The contrast of chemical
inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.Comment: 13 pages, 14 figure
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters
High-resolution spectropolarimetric observations provide simultaneous
information about stellar magnetic field topologies and three-dimensional
distributions of chemical elements. Here we present analysis of a unique full
Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap
star HD 24712. The goal of our work is to examine circular and linear
polarization signatures inside spectral lines and to study variation of the
stellar spectrum and magnetic observables as a function of rotational phase. HD
24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over
a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and
resolving power exceeding 100000. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the spectral lines
of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to
study the morphology of the Stokes profile shapes in individual spectral lines
and in LSD Stokes profiles corresponding to different line masks. From the LSD
Stokes V profiles we measured the longitudinal component of the magnetic field,
, with an accuracy of 5-10 G. We also determined the net linear
polarization from the LSD Stokes Q and U profiles. We determined an improved
rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured
from the cores of Halpha and Hbeta lines. The analysis of measurements
showed no evidence for a significant radial magnetic field gradient in the
atmosphere of HD 24712. We used our and net linear polarization
measurements to determine parameters of the dipolar magnetic field topology. We
found that magnetic observables can be reasonably well reproduced by the
dipolar model. We discovered rotational modulation of the Halpha core and
related it a non-uniform surface distribution of rare-earth elements.Comment: Accepted for publication in A&
No magnetic field in the spotted HgMn star mu Leporis
Chemically peculiar stars of the mercury-manganese (HgMn) type represent a
new class of spotted late-B stars, in which evolving surface chemical
inhomogeneities are apparently unrelated to the presence of strong magnetic
fields but are produced by some hitherto unknown astrophysical mechanism. The
goal of this study is to perform a detailed line profile variability analysis
and carry out a sensitive magnetic field search for one of the brightest HgMn
stars - mu Lep. We acquired a set of very high-quality intensity and
polarization spectra of mu Lep with the HARPSpol polarimeter. These data were
analyzed with the multiline technique of least-squares deconvolution in order
to extract information on the magnetic field and line profile variability. Our
spectra show very weak but definite variability in the lines of Sc, all Fe-peak
elements represented in the spectrum of mu Lep, as well as Y, Sr, and Hg.
Variability might also be present in the lines of Si and Mg. Anomalous profile
shapes of Ti II and Y II lines suggest a dominant axisymmetric distribution of
these elements. At the same time, we found no evidence of the magnetic field in
mu Lep, with the 3 sigma upper limit of only 3 G for the mean longitudinal
magnetic field. This is the most stringent upper limit on the possible magnetic
field derived for a spotted HgMn star. The very weak variability detected for
many elements in the spectrum mu Lep suggests that low-contrast chemical
inhomogeneities may be common in HgMn stars and that they have not been
recognized until now due to the limited precision of previous spectroscopic
observations and a lack of time-series data. The null result of the magnetic
field search reinforces the conclusion that formation of chemical spots in HgMn
stars is not magnetically driven.Comment: Accepted for publication in Astronomy & Astrophysic
Magnetism, chemical spots, and stratification in the HgMn star Ï• Phoenicis
Context. Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars
for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface
distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key
role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.
Aims. In this study, we investigate the possible presence of a magnetic field in Ï• Phe (HD 11753) and reconstruct surface distribution
of chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemical
spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.
Methods. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of
the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field
search, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructed
surface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elements
was calculated with the DDAFit program.
Results. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field.
For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti
and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric
layers.
Conclusions. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in Ï• Phe.
This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the
typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the
spot evolution time-scale reported for Ï• Phe
Are there tangled magnetic fields on HgMn stars?
Several recent spectrophotometric studies failed to detect significant global
magnetic fields in late-B HgMn chemically peculiar stars, but some
investigations have suggested the presence of strong unstructured or tangled
fields in these objects. We used detailed spectrum synthesis analysis to search
for evidence of tangled magnetic fields in high-quality observed spectra of 8
slowly rotating HgMn stars and one normal late-B star. We also evaluated recent
sporadic detections of weak longitudinal magnetic fields in HgMn stars based on
the moment technique. Our analysis of the Zeeman broadening of magnetically
sensitive spectral lines reveals no evidence of tangled magnetic fields in any
of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the
mean magnetic field modulus -- much smaller than the field strengths implied by
studies based on differential magnetic line intensification and quadratic field
diagnostics. The new HARPSpol longitudinal field measurements for the extreme
HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero
at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted
HgMn star HD 11753 shows that the recent moment technique measurements
retrieved from the same data are incompatible with the lack of circular
polarization signatures in the spectrum of this star. We conclude that there is
no evidence for substantial tangled magnetic fields on the surfaces of studied
HgMn stars. We cannot independently confirm the presence of very strong
quadratic or marginal longitudinal fields for these stars, so results from the
moment technique are likely to be spurious.Comment: 12 pages, 11 figures; accepted for publication in A&
Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae
Stars and planetary system
Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TQ Hydrae
We present high spectral resolution (R ≈ 108,000) Stokes V polarimetry of the classical T Tauri stars (CTTSs)
GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m
telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the
polarization properties of the He i emission lines at 5876 Ã… and 6678 Ã…. The He i lines in these CTTSs contain both
narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission
components which may come from either a wind or the large-scale magnetospheric accretion flow.We detect strong
polarization in the narrow component of the two He i emission lines in both stars. We observe a maximum implied
field strength of 6.05 ± 0.24 kG in the 5876 Å line of GQ Lup, making it the star with the highest field strength
measured in this line for a CTTS. We find field strengths in the two He i lines that are consistent with each other,
in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the
broad component of the He i lines on these stars, strengthening the conclusion that they form over a substantially
different volume relative to the formation region of the narrow component of the He i lines
The search for magnetic fields in mercury-manganese stars
We performed a highly sensitive search for magnetic fields on a large set of
HgMn stars. With the aid of a new polarimeter attached to the HARPS
spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular
polarization spectra of 41 single and double HgMn stars. Using a multi-line
analysis technique on each star, we co-added information from hundreds of
spectral lines resulting in significantly greater sensitivity to the presence
of magnetic fields, including very weak fields. For the 47 individual objects
studied, including 6 components of SB2 systems, we do not detect any magnetic
fields at greater than the 3 sigma level. The lack of detection in the circular
polarization profiles indicates that if strong fields are present on these
stars, they must have complex surface topologies. For simple global fields, our
detection limits imply upper limits to the fields present of 2-10 Gauss in the
best cases. We conclude that HgMn stars lack large-scale magnetic fields,
typical for spotted magnetic Ap stars, sufficient to form and sustain the
chemical spots observed on HgMn stars. Our study confirms that in addition to
magnetically altered atomic diffusion, there exists another differentiation
mechanism operating in the atmospheres of late-B main sequence stars which can
compositional inhomogeneities on their surfaces.Comment: 12 pages, 8 figures, 2 table