Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf B-star pulsatos
Summary (2 min read)
- Key words: surveys – binaries (including multiple): close – stars: oscillations (including pulsations) – white dwarfs.
- Recently, a new class of short-period pulsating hot stars known as Blue Large-Amplitude Pulsators was discovered by Pietrukowicz et al. (2017).
- Table 1. Overview of the high-cadence Galactic plane survey.
- Because more time was available each night most fields were observed for ≈3 h.
2.1 Field selection
- The main science driver for the survey is to find and study UCBs consisting of fully degenerate or semi-degenerate stars.
- Hence, the authors selected ZTF fields based on the density of objects residing well below the main sequence, including mostly white dwarfs and hot subdwarf stars.
- To achieve this, the authors extracted objects with absolute magnitudes which placed them below the main-sequence based on Gaia data release 2 (Gaia Collaboration et al. 2016, 2018).
- Only objects with declinations >−30◦ were selected.
- For each object, the ZTF field was calculated and the fields with the largest number of individual objects were selected for their survey.
2.2 Data-processing and light-curve extraction
- Data-processing and light-curve generation follows the standard procedure for ZTF and occurs at the Infrared Processing and Analysis Center, Caltech.
- Each reference image is constructed using a minimum of 15 and maximum of 40 good-quality epochal images, yielding depths of ≈2–2.5 mag deeper than the single-epoch images.
- The grey shaded region corresponds to the underlying Hertzsprung– Russell diagram showing the position of the main sequence and the red giant branch.
- Further details of the data processing, PSF-fit photometry software, light-curve generation, formats, and overall performance on photometric accuracy are described in Masci et al. (2019).
- Fig. 3 shows the sky density of the extracted objects overplotted with the fields observed as part of the high-cadence Galactic plane survey.
3 R ESULTS FROM REPRESENTATIVE FIELDS
- The authors select four representative fields with different stellar densities and calculate statistics for each individual light curve, including median magnitude, reduced χ2, interquartile range (IQR), skewness, inverse von-Neumann, Stetson J, and Stetson K statistics.
- The two white square areas in FieldID 331 are individual quadrants, which were not processed.
- Histogram of the IQR extracted at a ZTF-r median magnitude between 15.5 and 16 with the best Gaussian fit and the limit above which an object is called variable.
- The grey shaded region corresponds to the underlying distribution of all objects in that field.
- The authors find that out of the selected light curves only 15 per cent for FieldID = 331 and 40 per cent for FieldID = 309 show real astrophysical variability which shows that in the higher density fields the contamination of false positives is significantly larger.
4 PRO G R E S S R E P O RT
- The primary science driver for the survey is the detection and study of ultracompact binaries and rapid pulsators.
- Because the data volume is too large to inspect every single light curve, the authors applied different cuts to pre-select light curves in early analysis.
- As part of their cross-match with the hot subdwarf catalogue, the authors discovered ZTF J2130, the most compact sdB+WD binary and the first member of systems where the sdB fills its.
- The typical diurnal mean sampling rate of ZTF is too short to resolve the pulsations for many pulsating white dwarfs (ZZ Ceti pulsation modes range from 100–1500 sec Mukadam et al. 2013; Bognár et al. 2020).
- Using the first data release from the 14 August fields, more than 1500 stellar flare events have been detected (Klein et al., in preparation).
5 EARLY R ESULTS – N EW SINGLE-MODE HOT SUB DWARF PULSATORS
- Subdwarf B stars (sdBs) are hot stars of spectral type B with luminosities below the main sequence.
- The authors discovered 12 new sdB pulsators that show only a single pulsation mode as part of this survey.
- The authors have presented an overview and first results of their dedicated high-cadence Galactic plane survey for short-period variable objects, carried out as part of ZTF during its first year of operation.
- The authors calculate light curve statistics for each field and use the IQR to estimate the number of variable objects for each field.
- This research was funded by the Gordon and Betty Moore Foundation through Grant GBMF5076.
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Q1. What are the contributions in "Year 1 of the ztf high-cadence galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf b-star pulsatos" ?
The authors present the goals, strategy, and first results of the high-cadence Galactic plane survey using the Zwicky Transient Facility ( ZTF ). As part of this survey, the authors extract a total of ≈230 million individual objects with at least 80 epochs obtained during the high-cadence Galactic plane survey reaching an average depth of ZTF–r ≈ 20. The authors present a progress report on recent discoveries, including a new class of compact pulsators, the first members of a new class of Roche lobe filling hot subdwarf binaries as well as new ultracompact double white dwarfs and flaring stars. Finally, the authors present a sample of 12 new single-mode hot subdwarf B-star pulsators with pulsation amplitudes between ZTF–r = 20–76 mmag and pulsation periods between P = 5.