20 research outputs found
Impact of Level 1 quality on BIRRA CO retrieval
No full textThe SCIAMACHY near infrared (NIR) channels 6-8 are suitable to
retrieve vertical column densities (VCDs) of atmospheric gases such as
CO, CH4, CO2 or H2O. BIRRA will be used in the coming version 5 of
the offline Level 1b to 2 SCIAMACHY processor to retrieve CO values
operationally.
The quality of VCD retrievals depends on theoretical, computational
and observational issues. Realistic atmospheric data as
well as efficient yet accurate, robust radiation transfer models
and inversion methods have to be used. However, especially operational
applications require to keep computational time within strict limits
and the best possible models are chosen according to this restriction.
The quality of the measured spectra greatly affects the accuracy of the
retrievals. Since model parameters are optimally varied during the inversion
process to mimic the measured values, errors in the input spectra will
lead to wrong retrievals.
SCIAMACHY spectra are spectrally and radiometrically calibrated and
corrected for several effects, namely: Leakage current,
pixel-to-pixel gain, non-linear response, straylight, polarisation.
Reflectances are calculated using inflight sun diffuser spectra. Additionally,
the degradation of the instrument is being monitored and the quality of the
individual spectral pixels assessed. It turned out that the effect of the
bad/dead pixel mask (BDPM) has a major impact on the retrievals.
Focussing on carbon monoxide, this work aims at studing the effect of
different calibration steps on the retrieval of atmospheric gas
VCDs
Single magnetic white dwarfs with Balmer emission lines: A small class with consistent physical characteristics as possible signposts for close-in planetary companions
No full textWe report the identification of SDSS J121929.45+471522.8 as the third apparently isolated magnetic (B ≃ 18.5 ± 1.0 MG) white dwarf exhibiting Zeeman-split Balmer emission lines. The star shows coherent variability at optical wavelengths with an amplitude of ≃0.03 mag and a period of 15.26 h, which we interpret as the spin period of the white dwarf. Modelling the spectral energy distribution and Gaia parallax, we derive a white dwarf temperature of 7500 ± 148 K, a mass of 0.649 ± 0.022 M⊙, and a cooling age of 1.5 ± 0.1 Gyr, as well as an upper limit on the temperature of a sub-stellar or giant planet companion of ≃250 K. The physical properties of this white dwarf match very closely those of the other two magnetic white dwarfs showing Balmer emission lines: GD356 and SDSS J125230.93-023417.7. We argue that, considering the growing evidence for planets and planetesimals on close orbits around white dwarfs, the unipolar inductor model provides a plausible scenario to explain the characteristics of this small class of stars. The tight clustering of the three stars in cooling age suggests a common mechanism switching the unipolar inductor on and off. Whereas Lorentz drift naturally limits the lifetime of the inductor phase, the relatively late onset of the line emission along the white dwarf cooling sequence remains unexplained
ASAS J071404+7004.3 -- a close, bright nova-like with gusty winds
No full textDespite being bright () and nearby ( pc) ASAS J071404+7004.3 has only recently been identified as a nova-like cataclysmic variable. We present time-resolved optical spectroscopy obtained at the Isaac Newton Telescope together with X-ray and ultraviolet observations. We combined these with photometry and find a period of 3.27h and a mass transfer rate of . Historical photometry shows at least one low state establishing the system as a VY Scl star. Our high-cadence spectroscopy also revealed rapidly changing winds emanating from the accretion disc. We have modelled these using the Monte Carlo PYTHON code and shown that all the emission lines could emanate from the wind - which would explain the lack of double-peaked lines in such systems. In passing,we discuss the effect of variability on the position of cataclysmic variables in the Hertzsprung-Russell diagram
Cataclysmic Variables from Sloan Digital Sky Survey - V. The search for period bouncers continues
No full textSDSS-V is carrying out a dedicated survey for white dwarfs, single, and in binaries, and we report the analysis of the spectroscopy of 118 cataclysmic variables (CVs) and CV candidates obtained during the final plug plate observations of SDSS. We identify eight new CVs, spectroscopically confirm 53 and refute 11 published CV candidates, and we report 21 new or improved orbital periods. The orbital period distribution of the SDSS-V CVs does not clearly exhibit a period gap. In common with previous studies, the distribution shows that spectroscopically identified CVs have a larger proportion of short-period systems compared to samples identified from photometric variability. Remarkably, despite a systematic search, we find very few period bouncers. We estimate the space density of period bouncers to be, i.e. they represent only a few per cent of the total CV population. This suggests that during their final phase of evolution, CVs either destroy the donor, e.g. via a merger, or that they become detached and cease mass transfer
Long-term variability in debris transiting white dwarfs
No full textCombining archival photometric observations from multiple large-area surveys spanning the past 17 years, we detect long-term variability in the light curves of ZTF J032833.52−121945.27 (ZTF J0328−1219), ZTF J092311.41+423634.16 (ZTF J0923+4236), and WD 1145+017, all known to exhibit transits from planetary debris. ZTF J0328−1219 showed an overall fading in brightness from 2011 through to 2015, with a maximum dimming of ≃0.3 mag, and still remains ≃0.1 mag fainter compared to 2006. We complement the analysis of the long-term behaviour of these systems with high-speed photometry. In the case of ZTF J0923+4236 and WD 1145+017, the time-series photometry exhibits vast variations in the level of transit activity, both in terms of numbers of transits, as well as their shapes and depths, and these variations correlate with the overall brightness of the systems. Inspecting the current known sample of white dwarfs with transiting debris, we estimate that similar photometric signatures may be detectable in one in a few hundred of all white dwarfs. Accounting for the highly aligned geometry required to detect transits, our estimates imply that a substantial fraction of all white dwarfs exhibiting photospheric metal pollution from accreted debris host close-in planetesimals that are currently undergoing disintegration
Cataclysmic Variables from Sloan Digital Sky Survey – V. The search for period bouncers continues
No full textSDSS-V is carrying out a dedicated survey for white dwarfs, single, and in binaries, and we report the analysis of the spectroscopy of 118 cataclysmic variables (CVs) and CV candidates obtained during the final plug plate observations of SDSS. We identify eight new CVs, spectroscopically confirm 53 and refute 11 published CV candidates, and we report 21 new or improved orbital periods. The orbital period distribution of the SDSS-V CVs does not clearly exhibit a period gap. In common with previous studies, the distribution shows that spectroscopically identified CVs have a larger proportion of short-period systems compared to samples identified from photometric variability. Remarkably, despite a systematic search, we find very few period bouncers. We estimate the space density of period bouncers to be , i.e. they represent only a few per cent of the total CV population. This suggests that during their final phase of evolution, CVs either destroy the donor, e.g. via a merger, or that they become detached and cease mass transfer.https://arxiv.org/abs/2305.13371First author draf
A catalogue of cataclysmic variables from 20 years of the Sloan Digital Sky Survey with new classifications, periods, trends and oddities
No full textWe present a catalogue of 507 cataclysmic variables (CVs) observed in SDSS I
to IV including 70 new classifications collated from multiple archival data
sets. This represents the largest sample of CVs with high-quality and
homogeneous optical spectroscopy. We have used this sample to derive unbiased
space densities and period distributions for the major sub-types of CVs. We
also report on some peculiar CVs, period bouncers and also CVs exhibiting large
changes in accretion rates. We report 70 new CVs, 59 new periods, 178
unpublished spectra and 262 new or updated classifications. From the SDSS
spectroscopy, we also identified 18 systems incorrectly identified as CVs in
the literature. We discuss the observed properties of 13 peculiar CVS, and we
identify a small set of eight CVs that defy the standard classification scheme.
We use this sample to investigate the distribution of different CV sub-types,
and we estimate their individual space densities, as well as that of the entire
CV population. The SDSS I to IV sample includes 14 period bounce CVs or
candidates. We discuss the variability of CVs across the Hertzsprung-Russell
diagram, highlighting selection biases of variability-based CV detection.
Finally, we searched for, and found eight tertiary companions to the SDSS CVs.
We anticipate that this catalogue and the extensive material included in the
Supplementary Data will be useful for a range of observational population
studies of CVs.Comment: Published by MNRAS (including 2 corrections published on 7 December
2023). Includes supplementary material including machine readable versio
DAHe white dwarfs from the DESI survey
No full textA new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer
lines in emission has recently emerged. However, the physical origin of these
emission lines remains unclear. We present here a sample of 21 newly identified
DAHe systems and determine magnetic field strengths and (for a subset) periods
which span the ranges of ~ 6.5 -- 147 MG and ~ 0.4 -- 36 h respectively. All
but four of these systems were identified from the Dark Energy Spectroscopic
Instrument (DESI) survey sample of more than 47000 white dwarf candidates
observed during its first year of observations. We present detailed analysis of
the new DAHe WDJ161634.36+541011.51 with a spin period of 95.3 min, which
exhibits an anti-correlation between broadband flux and Balmer line strength
that is typically observed for this class of systems. All DAHe systems cluster
closely on the Gaia Hertzsprung-Russell diagram where they represent ~ 1 per
cent of white dwarfs within that region. This grouping further solidifies their
unexplained emergence at relatively late cooling times and we discuss this in
context of current formation theories. Nine of the new DAHe systems are
identifiable from SDSS spectra of white dwarfs that had been previously
classified as featureless DC-type systems. We suggest high S/N, unbiased
observations of DCs as a possible route for discovering additional DAHe
systems.Comment: 19 pages, 10 Figures, accepted for publication in MNRA
Cataclysmic variables from Sloan Digital Sky Survey – V (2020–2023) identified using machine learning
No full textSDSS-V is carrying out a dedicated survey for white dwarfs, single and in binaries, and we report the analysis of the spectroscopy of 504 cataclysmic variables (CVs) and CV candidates obtained during the first 34 months of observations of SDSS-V. We developed a convolutional neural network (CNN) to aid with the identification of CV candidates among the over 2 million SDSS-V spectra obtained with the BOSS spectrograph. The CNN reduced the number of spectra that required visual inspection to per cent of the total. We identified 776 CV spectra among the CNN-selected candidates, plus an additional 27 CV spectra that the CNN misclassified, but that were found serendipitously by human inspection of the data. Analysing the SDSS-V spectroscopy and ancillary data of the 504 CVs in our sample, we report 61 new CVs, spectroscopically confirm 248 and refute 13 published CV candidates, and we report 82 new or improved orbital periods. We discuss the completeness and possible selection biases of the machine learning methodology, as well as the effectiveness of targeting CV candidates within SDSS-V. Finally, we re-assess the space density of CVs, and find
