Journal ArticleDOI

# Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf B-star pulsatos

07 Jun 2021-Monthly Notices of the Royal Astronomical Society (Oxford University Press (OUP))-Vol. 505, Iss: 1, pp 1254-1267

AbstractWe present the goals, strategy and first results of the high-cadence Galactic plane survey using the Zwicky Transient Facility (ZTF). The goal of the survey is to unveil the Galactic population of short-period variable stars, including short period binaries and stellar pulsators with periods less than a few hours. Between June 2018 and January 2019, we observed 64 ZTF fields resulting in 2990 deg$^2$ of high stellar density in ZTF-$r$ band along the Galactic Plane. Each field was observed continuously for 1.5 to 6 hrs with a cadence of 40 sec. Most fields have between 200 and 400 observations obtained over 2-3 continuous nights. As part of this survey we extract a total of $\approx$230 million individual objects with at least 80 epochs obtained during the high-cadence Galactic Plane survey reaching an average depth of ZTF-$r$ $\approx$20.5 mag. For four selected fields with 2 million to 10 million individual objects per field we calculate different variability statistics and find that $\approx$1-2% of the objects are astrophysically variable over the observed period. We present a progress report on recent discoveries, including a new class of compact pulsators, the first members of a new class of Roche Lobe filling hot subdwarf binaries as well as new ultracompact double white dwarfs and flaring stars. Finally we present a sample of 12 new single-mode hot subdwarf B-star pulsators with pulsation amplitudes between ZTF-$r$ = 20-76 mmag and pulsation periods between $P$ = 5.8-16 min with a strong cluster of systems with periods $\approx$ 6 min. All of the data have now been released in either ZTF Data Release 3 or data release 4.

Topics: Galactic plane (58%), Subdwarf B star (57%), Subdwarf (55%), Population (51%), Variable star (51%)

### Introduction

• 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.

Did you find this useful? Give us your feedback

Content maybe subject to copyright    Report

MNRAS 505, 1254–1267 (2021) https://doi.org/10.1093/mnras/stab1344
Advance Access publication 2021 May 12
Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and
early results on new single-mode hot subdwarf B-star pulsatos
Thomas Kupfer,
1,2
Thomas A. Prince ,
3
Jan van Roestel,
3
Eric C. Bellm,
4
Lars Bildsten,
5,1
Michael W. Coughlin ,
6
Andrew J. Drake,
3
Matthew J. Graham ,
3
Courtney Klein ,
7
Shrinivas R. Kulkarni,
3
Frank J. Masci,
8
Richard Walters,
9
Igor Andreoni,
3
Rahul Biswas ,
10
2
Dmitry A. Duev ,
3
Richard Dekany,
9
Joseph A. Guidry,
11
J. J. Hermes,
12
Russ R. Laher
8
and Reed Riddle
9
1
Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
2
Department of Physics and Astronomy, Texas Tech University, PO Box 41051, Lubbock, TX 79409, USA
3
Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
4
DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
5
Department of Physics, University of California, Santa Barbara, CA 93106, USA
6
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
7
Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
8
IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
9
Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
10
Department of Physics, The Oskar Klein Center, S tockholm University, AlbaNova, SE-10691 Stockholm, Sweden
11
Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
12
Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
Accepted 2021 May 5. Received 2021 May 5; in original form 2021 January 14
ABSTRACT
We present the goals, strategy, and ﬁrst results of the high-cadence Galactic plane survey using the Zwicky Transient Facility
(ZTF). The goal of the survey is to unveil the Galactic population of short-period variable stars, including short-period binaries,
and stellar pulsators with periods less than a few hours. Between 2018 June and 2019 January, we observed 64 ZTF ﬁelds resulting
in 2990 deg
2
of high stellar density in the ZTF-r band along the Galactic plane. Each ﬁeld was observed continuously for 1.5 to
6 h with a cadence of 40 sec. Most ﬁelds have between 200 and 400 observations obtained over 2–3 continuous nights. As part
of this survey, we 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 20.5 mag. For four selected ﬁelds with 2–10 million individual
objects per ﬁeld, we calculate different variability statistics and ﬁnd that 1–2 per cent of the objects are astrophysically variable
over the observed period. We 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, we 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 = 5.8–16 min with a strong cluster of systems with periods 6 min. All
of the data have now been released in either ZTF Data Release 3 or Data Release 4.
Key words: surveys binaries (including multiple): close stars: oscillations (including pulsations) white dwarfs.
1 INTRODUCTION
Large-scale optical time-domain surveys have opened a new window
to study the variable sky providing hundreds to thousands of epochs
across the whole sky. Starting with the Sloan Digital Sky Survey
(SDSS; York et al. 2000), a new generation of wide-ﬁeld optical
surveys has exploited new affordable CCD detectors to open the
frontier of data-intensive astronomy. This allows the study of stellar
variability on different time-scales across the full magnitude range.
In particular, surveys covering also low Galactic latitudes are well
suited to study the Galactic distribution of photometric variable stars.
E-mail: tkupfer@ttu.edu
Ground-based surveys include the Optical Gravitational Lensing
Experiment (OGLE; e.g. Soszy
´
nski et al. 2015),PTF(Lawetal.
2009), the Vista Variables in the Via Lactea (; Saito et al. 2012),
ASAS-SN, (Shappee et al. 2014; Jayasinghe et al. 2018), and most
recently ATLAS (Heinze et al. 2018; Tonry et al. 2018). In particular,
the fastest stellar variabilities on time-scales of minutes to hours are
of great interest for a large number of scientiﬁc questions. This
includes ultracompact binaries (UCBs), compact pulsators as well as
fast ﬂaring stars.
UCBs are a class of binary stars with orbital periods less than
about 60 min, consisting of a neutron star (NS)/white dwarf (WD)
primary and a Helium-star (He-star)/WD/NS secondary. These UCBs
are sources of low-frequency gravitational wave signals as probed
C
2021 The Author(s)

ZTF high-cadence Galactic plane survey 1255
by the Laser Interferometer Space Antenna (LISA) and are crucial to
our understanding of compact binary evolution and offer pathways
towards Type Ia and other thermonuclear supernovae. Systems with
orbital periods <20 min will be the strongest Galactic LISA sources
and will be detected by LISA within weeks after its operation begins
(Nelemans, Yungelson & Portegies Zwart 2004;Str
¨
oer & Vecchio
2006;Nissankeetal.2012; Littenberg et al. 2013; Korol et al. 2017;
Kremer et al. 2017; Kupfer et al. 2018; Lamberts et al. 2019; Burdge
et al. 2019, 2020a) and as such are ideal multimessenger sources
(Shah, van der Sluys & Nelemans 2012; Shah & Nelemans 2014;
Baker et al. 2019; Littenberg et al. 2019; Kupfer et al. 2019a)
Short time-scale photometric variations can also originate from
astrophysical changes within the internal structure or atmosphere
of the star. Such sources are ﬂaring stars, pulsating stars, or white
dwarfs. Prominent examples of rapidly pulsating stars are δ Scuti
stars (Breger 2000), SX Phoenicis variables (Nemec & Mateo 1990),
or Ap or Am stars (Renson, Gerbaldi & Catalano 1991), and pulsating
white dwarfs (ZZ Ceti variables; Fontaine & Brassard 2008) as well
as pulsating extremely low-mass white dwarfs (Hermes et al. 2013).
The stars exhibit pulsation amplitudes of less than 1 percent up to
several tens of percent on time-scales of few to tens of minutes.
Recently, a new class of short-period pulsating hot stars known
as Blue Large-Amplitude Pulsators (BLAPs) was discovered by
Pietrukowicz et al. (2017). Their pulsation periods are typically
between 20–40 min. Romero et al. (2018) and Byrne & Jeffery (2018)
proposed that the BLAPs are hot pre-helium white dwarfs that are
cooling and contracting, with masses in the range 0.3–0.35 M
.A
recent study by Meng et al. (2020) suggests that BLAPs could be
the surviving companions of type Ia supernovae. In this scenario
mass transfer from the BLAP progenitor leads to the detonation of a
white dwarf companion, leaving behind a BLAP as single star. Their
pulsation properties can best be explained by fundamental radial
mode pulsators. However, the known sample of BLAPs is small and
in-homogeneous, discovered only in the OGLE survey.
A number of fast cadence ground-based surveys have been
executed to study the variable sky down to a few minutes period. The
main goal of these high-cadence surveys is the discovery of rapid
brightness variations seen in UCBs, compact pulsators as well as fast
ﬂaring stars. The ﬁrst survey at low Galactic latitudes targeting short-
period systems was the Rapid Temporal Survey (RATS; Ramsay
& Hakala 2005; Barclay et al. 2011) covering a total of 46 deg
2
.
Another more recent survey is the OmegaWhite (OW) survey, which
covers a total of 400 deg
2
at low Galactic latitudes (|b| < 10
)aswell
as in the Galactic Bulge using high-cadence optical observations.
Two neighbouring 1 deg
2
ﬁelds are alternatingly observed in 39-s
exposures over an observing duration of 2 h, with an observational
median cadence of 2.7 min per ﬁeld (Macfarlane et al. 2015,
2017a,b; Toma et al. 2016; Kupfer et al. 2017)
As part of the Zwicky Transient Facility (ZTF), the Palomar 48-
inch (P48) telescope images the sky every clear night conducting
several surveys including a Northern Sky Survey with a 3-d cadence,
as well as smaller surveys such as a 1-d Galactic plane survey and
simultaneous observations of the Northern TESS sectors (Graham
et al. 2019; van Roestel et al. 2019; Bellm et al. 2019a,b). As part of
the partnership share of ZTF, we conducted a dedicated high-cadence
Galactic plane survey with a cadence of 40 sec at low Galactic
latitudes aiming to ﬁnd UCBs and compact pulsators. During that
dedicated survey, we either observed one ﬁeld or alternated between
two adjacent ﬁelds continuously for 1.5–3 h on two to three
consecutive nights in the ZTF-r band. Here, we present an overview
of the ZTF high-cadence Galactic plane survey executed in ZTF year
1. We present the observing strategy as well as some survey statistics.
Tab le 1 . Overview of the high-cadence Galactic plane survey.
Period No. of ﬁelds Sky coverage Filter
(deg
2
)
06-15-2018–07-31-2018 16 750 ZTF-r
08-03-2018–08-18-2018 14 650 ZTF-r
11-15-2018–01-15-2019 34 1590 ZTF-r
We show a progress report and ﬁnalize with some early results from
the survey. In Section 2, we discuss the observing strategy as well
as the ﬁeld selection and the data processing. In Section 3, we
present results for four representative ﬁelds and in Section 4 we give
a progress report of already published results. In Section 5, we show
some new early results from the high-cadence Galactic plane survey
and summarize and conclude in Section 6.
GALACTIC PLANE SURVEY
ZTF uses a 47 deg
2
camera consisting of 16 individual CCDs each 6k
× 6k covering the full focal plane of the P48 telescope. The ZTF high-
cadence Galactic plane survey covered a total of 2990 deg
2
split
in 64 individual ZTF ﬁelds observed over three observing blocks;
mid-June–July 2018, two weeks in 2018 August and November 15
to 2019 January 15. All observations of the high-cadence Galactic
plane survey were obtained in ZTF-r band. Each ﬁeld has 200–
400 epochs. The images were processed using ZTF data-processing
pipeline described in full detail in Masci et al. (2019).
During June and July, we observed 16 ﬁelds covering 750 deg
2
mostly at low Galactic longitudes. In June, we observed every ﬁeld
continuously for 1 h15 min and in July for 1 h 25 min. All ﬁelds
were observed over 2 or 3 consecutive nights. In August, 14 ﬁelds
covering 650 deg
2
were observed over two weeks (see Table 1).
We alternated between two adjacent ﬁelds continuously for 2 h
40min each night. The same ﬁelds were repeated the following night.
The observations in June/July and August were done under stable
conditions with an average seeing of 2 arcsec. We lost only a total of
ﬁve nights due to weather during June/July and August observations.
Between 2018 November 15 and 2019 January 15 high-cadence
Galactic plane observations were scheduled for every night. We
observed an additional 33 ﬁelds covering 1550 deg
2
with stable
weather conditions (see Table 1). The overall strategy varied during
those two months due to unstable weather. Most ﬁelds were observed
alternating with adjacent ﬁelds but in particular low declination
ﬁelds were observed continuously. Because more time was available
each night most ﬁelds were observed for 3 h. However, due to
the unstable weather conditions about half of the ﬁelds were only
observed in a single night and not repeated in subsequent nights. All
other ﬁelds were observed over 2 or 3 nights. Although the seeing
varies strongly between 1.7 and 4 arcsec, each ﬁeld has a limiting
magnitude of >19.5 mag. A detailed overview of the ﬁelds observed
in stable weather conditions is given in Tables A1A3.
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, we
selected ZTF ﬁelds based on the density of objects residing well
below the main sequence, including mostly white dwarfs and hot
subdwarf stars.
MNRAS 505, 1254–1267 (2021)

1256 T. Kupfer et al.
Figure 1. Sky density of candidate white dwarfs and hot subdwarfs selected from Gaia DR2 with the selected ﬁelds of the ZTF high-cadence Galactic plane
survey observed in ZTF year-1. The squares show individual ZTF ﬁelds which have been observed in high-cadence Galactic plane observations. The white lines
correspond to the Galactic equator and |b|=15
o
.
To achieve this, we 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 de-
clinations >30
were selected. As the goal was not to extract a clean
sample we used a very relaxed tolerance for the parallax precision
( /σ
> 3) and did not include any additional quality cuts, resulting
in 350 000 individual objects (Fig. 2). For each object, the ZTF ﬁeld
was calculated and the ﬁelds with the largest number of individual
objects were selected for our survey. Selected ﬁelds with the highest
density have 1000 selected objects per deg
2
. The ﬁnal ﬁeld selection
was mainly driven by stellar density but also included other aspects
like visibility and weather. Fig. 1 shows the sky density of the selected
sources overplotted with the ﬁelds observed as part of the high-
cadence Galactic plane survey. As the stellar density is highest at low
Galactic latitudes most ﬁelds are located at Galactic latitudes 15
o
.
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. The raw camera image data are ﬁrst instrumentally
calibrated and astrometric solutions are derived using the Gaia DR1
catalogue. Sources are detected and ﬂuxes measured using both aper-
ture (Bertin & Arnouts 1996) and PSF-ﬁt photometry (Stetson 1987).
Sources are photometrically calibrated using the Pan-STARRS1 DR1
catalogue. The epochal image data are then co-added within their
respective survey ﬁelds and camera readout channels to construct
reference images. 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 reference image co-adds are archived for use in other down-
stream processing: image differencing and light curve construction.
To support light curve generation, sources are ﬁrst detected and
Figure 2. Hertzsprung–Russell diagram of the objects used for the ﬁeld
selection. The grey shaded region corresponds to the underlying Hertzsprung–
Russell diagram showing the position of the main sequence and the red giant
branch. The colour–coded region corresponds to the objects which were
selected for the ﬁeld selection. The clump around M
G
5 corresponds to the
hot subdwarfs whereas the region below corresponds to the white dwarfs.
extracted from each reference image using PSF-ﬁt photometry
by running the DAOPhot utility (Stetson 1987). These sources
provide the seeds to facilitate positional cross-matching across all
the single-epoch-based PSF extraction catalogues going back to
the beginning of the survey. PSF-ﬁtting on the single-epoch images
is also performed using DAOPhot. These single-epoch images are
direct single exposure images, not difference images. A ﬁxed source-
match radius of 1.5 arcsec is used for the positional matching. Only
the PSF-ﬁt-derived positions and photometry, along with selected
PSF-ﬁt metrics are retained during the source-matching process.
Aperture photometry is not propagated to the light-curve metadata.
MNRAS 505, 1254–1267 (2021)

ZTF high-cadence Galactic plane survey 1257
Figure 3. Sky density of individual objects with at least 80 epochs from the ZTF high-cadence Galactic plane survey observed in ZTF year-1. The squares show
individual ZTF ﬁelds, which have been observed in high-cadence Galactic plane observations. The white lines correspond to the Galactic equator and |b|=15
o
.
Further details of the data processing, PSF-ﬁt photometry soft-
ware, light-curve generation, formats, and overall performance on
photometric accuracy are described in Masci et al. (2019). We
extracted the light curves from each object which has at least 80
epochs obtained as part of the high-cadence Galactic plane survey
from the ZTF light curve data base ‘Kowalski’ and extracted a
total of 230 million light curves across all observed ﬁelds. 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 RESULTS FROM REPRESENTATIVE FIELDS
We select four representative ﬁelds with different stellar densities and
calculate statistics for each individual light curve, including median
magnitude, reduced χ
2
, interquartile range (IQR), skewness, inverse
von-Neumann, Stetson J, and Stetson K statistics. We refer to table 2
in Coughlin et al. (2020) for a detailed description of the individual
statistics. These statistics are used to evaluate light curve variability
and identify variable objects in the high-cadence Galactic plane data.
The selected ﬁelds are FieldID = 331 (l=8.
59, b=8.
69),
FieldID = 538 (l = 44.
64, b = 3.
09), FieldID = 769
((l = 91.
87, b = 2.
38)l = 91.87 deg, b =−2.38 deg) and FieldID =
309 (l = 230.
68, b = 2.
98). Fig. 4 shows the stellar density of the
four selected ﬁelds. The different colouring correspond to different
densities. Some structure of lower and higher density regions can
be seen in the ﬁelds. The white circles are masked regions due to
saturated stars and the horizontal and vertical gaps are chip gaps
between the 16 CCDs. The two white square areas in FieldID
331 are individual quadrants, which were not processed. FieldID
= 331 was observed in three consecutive nights for 1.5 h each
night. FieldID = 538 was observed for two consecutive nights for
1.5 and 2.5 h. FieldID = 769 was observed for two consecutive
nights for 3 h per night and FieldID = 309 was observed for two
consecutive nights for 1.7 and 3 h. See Tables A1A3 for more
details. The four selected ﬁelds represent a large range of stellar
densities from 10 million individual light curves for FieldID =
331, 5.8 million for FieldID = 538, 3.8 million for FieldID =
769, and 2.3 million for FieldID = 309.
Different statistics can be used to evaluate whether an object
is considered variable or not. We decided to use the IQR which
corresponds to the difference between the upper and lower quartile
values. The main advantage of IQR is that it is a robust statistic not
affected by extreme outliers due to individual bad photometry data
points and therefore a better estimate of the general spread around the
median magnitude. When there are outliers in a sample, the median
and IQR are best used to summarize a typical value and the variability
in the sample, respectively. Large variability in the light curve leads
to a larger IQR.
To estimate whether an object is variable in the high-cadence
Galactic plane data, we used the following approach. We calculate the
median ZTF-r band magnitude for each object. The full magnitude
range of all objects in a ZTF ﬁeld is binned into several bins and each
object is added to a bin based on its median magnitude. We used bin
sizes of 0.5 mag between 12 and 16.5 mag where the IQR is constant
at 0.02 mag for non-variable sources. Above 16.5 mag, we used
bin sizes of 0.2 mag because the IQR for non-variable sources is
increasing with increasing magnitudes to 0.2 mag at the faint end
at ZTF-r = 20.5 mag because the underlying noise of the sources
is increasing for fainter sources. Each bin contains at least a few
thousand objects.
For each magnitude bin, we then calculate a histogram of IQR
values. If there are only constant sources with a similar noise pattern,
a normal distribution in an IQR histogram is expected. An excess
of variable light curves will result in a deviation from a normal
distribution towards more objects with larger IQR values. To estimate
the excess and number of variable objects, we ﬁt a Gaussian to each
IQR histogram and deﬁne an object as variable if, in a given histogram
bin, less than 5 per cent of the sources in that magnitude bin are
MNRAS 505, 1254–1267 (2021)

1258 T. Kupfer et al.
Tab le 2 . Photometric properties of the high-gravity-BLAPs.
Object RA (J2000) Dec. (J2000) P A
b
ZTF-r
c
g
a
g r
a
FieldID
(
h
:
min
:
sec
)(
:
:

) (s) (mmag) (mas) (mag) (mag)
ZTF-sdBV1 18:26:36.09 +10:00:22.3 347.287 ± 0.02 19.9 0.5640 ± 0.0372 14.90 0.41 538
ZTF-sdBV2 19:13:54.77 11:05:19.4 360.576 ± 0.02 19.6 0.4568 ± 0.0888 16.24 0.33 385
ZTF-sdBV3 21:03:08.50 +44:43 14.9 365.822 ± 0.01 53.3 0.3550 ± 0.0769 16.07 0.31 768
ZTF-sdBV4 08:06:07.05 20:08 19.1 370.481 ± 0.02 26.0 0.4021 ± 0.0531 16.37 0.32 311
ZTF-sdBV5 21:49:45.56 +45:58 36.5 375.690 ± 0.02 36.1 0.3112 ± 0.0683 16.55 0.35 770
ZTF-sdBV6 07:40:00.71 14:42 02.5 375.782 ± 0.02 21.1 0.5979 ± 0.0394 15.10 0.36 310
ZTF-sdBV7 19:24:08.62 12:58 30.9 377.468 ± 0.02 19.3 0.7399 ± 0.0355 14.64 0.41 385
ZTF-sdBV8 06:54:48.55 25:22 08.3 435.459 ± 0.02 31.4 0.5458 ± 0.0318 13.93 0.44 259
ZTF-sdBV9 18:52:49.07 18:46 08.4 524.829 ± 0.02 73.4 0.1427 ± 0.1538 17.12 0.38 334
ZTF-sdBV10 19:49:59.34 +08:31 06.1 646.306 ± 0.02 32.8 0.3884 ± 0.0642 16.23 0.38 541
ZTF-sdBV11 07:18:43.69 02:29 31.2 688.485 ± 0.02 30.2 0.1571 ± 0.1329 17.52 0.33 411
ZTF-sdBV12 17:07:41.34
15:22 42.9 995.479 ± 0.02 37.3 0.4376 ± 0.0346 14.11 0.35 330
a
From the Pan-STARRS release 1 (PS1) survey (Chambers et al. 2016) reddening corrected using Green et al. (2019).
b
Amplitude in ZTF-r from the ZTF light curves.
c
Parallaxes from Gaia EDR3 (Gaia Collaboration et al. 2016, 2020).
Figure 4. Sky density of ZTF objects with at least 80 epochs obtained during the high-cadence Galactic plane survey for four selected ﬁelds. The white circles
are masked regions due to saturated stars and the horizontal and vertical gaps are chip gaps between the 16 CCDs. The two white square areas in FieldID 331
are individual quadrants, which were not processed.
considered constant based on the Gaussian ﬁt. Fig. 5 shows two
examples of IQR histograms with Gaussian ﬁts of FieldID = 309
and 331 for the magnitude range 15.5–16 mag. The orange curve
corresponds to the Gaussian ﬁt of the underlying histogram and the
red-dashed line corresponds to the IQR limit above which we call an
object a variable.
Using this method, we ﬁnd 15 per cent variable sources for
FieldID = 331, 9 per cent variable sources for FieldID = 769, and
FieldID = 538 and 5 per cent variable sources for FieldID = 309 (see
Fig. 6). The number of variable sources per ﬁeld decreases with the
stellar density of the ﬁeld and it is likely that blending could produce
false positives where the object appears variable due to a close-by
MNRAS 505, 1254–1267 (2021)

##### Citations
More filters

Journal ArticleDOI
Abstract: AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-period systems (61.5 and 53.3 minutes) show many emission and absorption lines, indicating the presence of N, O, Na, Mg, Si, and Ca, and also the K and Zn, elements which have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is $\approx0.8$$\mathrm{M_{\odot}}$, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of $\approx$ 2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with He-star progenitors, although the observed spectral features seem to contradict this. The discovery of 5 new eclipsing AM~CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency. Based on this estimate, we expect to find another 1--4 eclipsing AM CVn systems as ZTF continues to obtain data. This will further increase our understanding of the population, but will require high precision data to better characterize these 5 systems and any new discoveries.

2 citations

Posted Content
Abstract: Binary systems of a hot subdwarf B (sdB) star + a white dwarf (WD) with orbital periods less than 2-3 hours can come into contact due to gravitational waves and transfer mass from the sdB star to the WD before the sdB star ceases nuclear burning and contracts to become a WD. Motivated by the growing class of observed systems in this category, we study the phases of mass transfer in these systems. We find that because the residual outer hydrogen envelope accounts for a large fraction of an sdB star's radius, sdB stars can spend a significant amount of time ($\sim$10s of Myr) transferring this small amount of material at low rates ($\sim 10^{-10}$-$10^{-9}\ M_\odot\,\rm yr^{-1}$) before transitioning to a phase where the bulk of their He transfers at much faster rates ($\gtrsim 10^{-8}\ M_\odot\,\rm yr^{-1}$). These systems therefore spend a surprising amount of time with Roche-filling sdB donors at orbital periods longer than the range associated with He star models without an envelope. We predict that the envelope transfer phase should be detectable by searching for ellipsoidal modulation of Roche-filling objects with $P_{\rm orb}=30$-$100$ min and $T_{\rm eff}=20{,}000$-$30{,}000$ K, and that many ($\geq$10) such systems may be found in the Galactic plane after accounting for reddening. We also argue that many of these systems may go through a phase of He transfer that matches the signatures of AM CVn systems, and that some AM CVn systems associated with young stellar populations likely descend from this channel.

1 citations

Journal ArticleDOI
26 Oct 2021

##### References
More filters

Journal ArticleDOI
Abstract: We present the automated techniques we have developed for new software that optimally detects, deblends, measures and classifies sources from astronomical images: SExtractor ( Source Extractor ). We show that a very reliable star/galaxy separation can be achieved on most images using a neural network trained with simulated images. Salient features of SExtractor include its ability to work on very large images, with minimal human intervention, and to deal with a wide variety of object shapes and magnitudes. It is therefore particularly suited to the analysis of large extragalactic surveys.

10,213 citations

Journal ArticleDOI
Abstract: The Sloan Digital Sky Survey (SDSS) will provide the data to support detailed investigations of the distribution of luminous and non- luminous matter in the Universe: a photometrically and astrometrically calibrated digital imaging survey of pi steradians above about Galactic latitude 30 degrees in five broad optical bands to a depth of g' about 23 magnitudes, and a spectroscopic survey of the approximately one million brightest galaxies and 10^5 brightest quasars found in the photometric object catalog produced by the imaging survey. This paper summarizes the observational parameters and data products of the SDSS, and serves as an introduction to extensive technical on-line documentation.

9,484 citations

Journal ArticleDOI
Donald G. York1, Jennifer Adelman2, John E. Anderson2, Scott F. Anderson3  +148 moreInstitutions (29)
Abstract: The Sloan Digital Sky Survey (SDSS) will provide the data to support detailed investigations of the distribution of luminous and nonluminous matter in the universe: a photometrically and astrometrically calibrated digital imaging survey of π sr above about Galactic latitude 30° in five broad optical bands to a depth of g' ~ 23 mag, and a spectroscopic survey of the approximately 106 brightest galaxies and 105 brightest quasars found in the photometric object catalog produced by the imaging survey. This paper summarizes the observational parameters and data products of the SDSS and serves as an introduction to extensive technical on-line documentation.

9,207 citations

Journal ArticleDOI
Abstract: Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims: A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods: The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results: Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330-680 nm) and GRP (630-1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions: Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

7,024 citations

Journal ArticleDOI
Abstract: The tasks of the DAOPHOT program, developed to exploit the capability of photometrically linear image detectors to perform stellar photometry in crowded fields, are discussed. Raw CCD images are prepared prior to analysis, and following the obtaining of an initial star list with the FIND program, synthetic aperture photometry is performed on the detected objects with the PHOT routine. A local sky brightness and a magnitude are computed for each star in each of the specified stellar apertures, and for crowded fields, the empirical point-spread function must then be obtained for each data frame. The GROUP routine divides the star list for a given frame into optimum subgroups, and then the NSTAR routine is used to obtain photometry for all the stars in the frame by means of least- squares profile fits. The process is illustrated with images of stars in a crowded field, and shortcomings and possible improvements of the program are considered.

4,747 citations