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A&A
Volume 619, November 2018
Article Number A157
Number of page(s) 7
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/201834004
Published online 20 November 2018

© ESO 2018

1 Introduction

All the gas giants in the solar system have multiple moons in orbit around them, and their nature gives insight into the earliest processes of planet and moon formation in the first 10−50 Myr of the solar system (e.g. see reviews by Armitage 2011; Kley & Nelson 2012). Planets presumably form through accretion within circumstellar disks, whereas most moons orbiting giant planets are thought to form from accretion of material within circumplanetary disks. The circumplanetary disk subsequently accretes onto the planet or into moons (Canup & Ward 2002; Magni & Coradini 2004; Ward & Canup 2010). Exomoons are expected to be common around giant exoplanets, and some projects are already under way searching for the signs of large satellites. The Hunt for Exomoons with Kepler (HEK) survey has searched for satellites around exoplanets in the Kepler data since 2011 (Kipping et al. 2012), yielding only one marginal candidate for a potential exomoon (Teachey & Kipping 2018) and placing strong upper limits on large moons orbiting short period (P < 20 d) exoplanets (Teachey et al. 2018). The possibility of tidal heating of exomoons in Laplace resonances (Peters & Turner 2013), combined with heating from the central planet, suggest that habitable zones can exist around giant exoplanets (Heller et al. 2014). The direct detection of an exo-circumplanetary disk presents the opportunity to study both the dynamics and composition of the forming planet.

The young star 1SWASP J140747.93-394542.6, recently given the variable star designation V1400 Cen1 (and hereafter referred to as J1407), is a probable kinematic member of the Sco–Cen OB Association (Mamajek et al. 2012; Pecaut & Mamajek 2016). J1407 is a very active, fast-rotating (~3 day) Li-rich K5 pre-main sequence star with estimated mass of ~0.9 M⊙. The Gaia Data Release 2 (Gaia Collaboration 2016, 2018) provides the first precise trigonometric parallax for V1400 Cen of ϖ = 7.1835 ± 0.0447 mas, consistent with d = 139.2 ± 0.8 pc2. We have revised the distance, brightness, age, and mass of the star using these new parameters and provided them in Table 1. A complex sequence of deep eclipses of J1407 in time-series photometry data from May 2007 was reported by Mamajek et al. (2012) and interpreted being as due to a large ring-like structure of debris, some 200 times larger than Saturn’s rings (Mamajek et al. 2012; van Werkhoven et al. 2014; Kenworthy & Mamajek 2015). The photometric variability during the transit ranges on timescales from hours through to months, suggesting a large coherent structure on the order of ~1 AU in diameter. The leading hypothesis is of an unseen secondary companion providing the gravity well in which this structure is resident.

To date, a search for the secondary companion through direct imaging has placed upper limits on the orbital period and mass (Kenworthy et al. 2015). Subsequent photometric monitoring has not detected a second eclipse, and photometry from wide field surveys using large format imaging cameras from the past 15 yr shows no other eclipse.

Over the past decade, archival photographic plates from DASCH (USA) as well as the Sonneberg Observatory and Bamberg Observatoryin Germany, have been digitized and light curves for all detected objects are now publicly available. J1407 is detected on about 1000 photographic plates that cover a baseline of over one century. In this paper, we present the photometricanalysis of these photographic plates in search of one or more additional eclipses, and include the latest photometry from several ongoing surveys. The photographic plate photometry of J1407 is presented for the first time.

In Sect. 2, we describe the photographic plate archives used for the search. In Sect. 3, we describe the search and analysis of the photographic plates. We describe the code we use to infer limits on the orbital period of the companion from the epochs of observation. In Sect. 5, we place the results in context of other measurements of the J1407 system.

Table 1

Stellar parameters of J1407 from (1): Zacharias et al. (2010), (2): Mamajek et al. (2012), newly calculated with data from (3): the Gaia Data Release 2, and from (4): Kenworthy et al. (2015).

2 Observations

We describe the sources of photometric data used in our analysis, along with Table 2 that details the photometric observations from photographic plates.

2.1 Photographic plates from DASCH

The Harvard College Observatory undertook an all-sky photographic survey of both the southern and northern hemisphere with various telescopes between 1885 and 1993, producing ~ 500 000 photographic plates, which are being digitized by the ongoing project Digital Access to a Sky Century @Harvard (DASCH; Tang et al. 2013).

From the b-series, 45 were recorded between 1890 and 1950 with a 8′′ Bache Doublet, Voigtlander at Mount Harvard in Colorado, Arequipa in Peru and Bloemfontein in South Africa. Another 350 plates from the am-series were taken between 1901 and 1926 using a 1′′ and 1.5′′ Cooke Lens in Arequipa, Peru, and Bloemfontein, SA. In 1909 and 1910, four plates from the ak-series were recorded with a 1.5′′ Cooke Long Focus in Hanover, SA. Another 138 plates in the mf-series were taken between 1919 and 1949 with a 10′′ Metcalf triplet in Arequipa, Peru and Bloemfontein, SA. A set of 112 plates in the ax-series were recorded between 1924 and 1948 using a 3′′ Ross-Tessar lens in Arequipa, Peru and Bloemfontein, SA and 215 plates in the rb-series were recorded between 1929 and 1951 with a 3′′ Ross-Fecker in Bloemfontein, SA. An additional 125 plates in the dsr-, dsy- and dsb-series were taken with three Damon telescopes equipped with red filter (9 plates), a yellow filter (9 plates and a blue filter (106 plates) in 1970 and 1971 in Bloemfontein, SA (dsr, dsy and dsb) as well as between 1978 and 1989 on Cerro Tololo, Chile and Mount John, New Zealand (dsb).

For our analysis (see Fig. 2) we used 989 photographic plates from the DASCH archive, observed between 1890 and 1989. These are made up of 602 plates whose photometry we acquired via the DASCH pipeline, and 387 plates which were not processed through the pipeline. The photometry pipeline is described in Tang et al. (2013), whose results can be accessed in a database on their website3. The plates from the pipeline have photometric errors that range from 0.04 to 0.4 mag with a median error of 0.14 mag, none of which show a sign of an ongoing eclipse.

Eighteen plates from the pipeline were taken with a red or yellow filter, recorded pairwise at identical epochs with plates with a blue filter to form RYB-triplets at single epochs. Since a single observation at an arbitrary epoch suffices to exclude a test period (see Sect. 3 for a detailed description), and all plates not accessible via the pipeline were taken with a blue filter, we restrict our photometric analysis of the photographic plates to those taken with a blue filter.

We analysed the other set of plates visually to confirm either the existence or the absence of a transit, and rejected 335 plates which failed to provide reliable photometry of J1407, leaving 52 plates for further analysis. The visually inspected plates consisted of ~ 100 px × 100 px sections with J1407 at its centre, which were too small to conduct a colour correction and automated photometry. A well recorded photographic plate from a sufficient aperture (>4 cm) usually has a sufficient sensitivity to detect a drop in brightness of 0.5 magnitudes. In several cases, artefacts like blurred and malformed stars or the sky noise prevented an accurate flux measurement of J1407. The remaining plates were used for the search of possible orbital periods.

We compared the brightness of J1407 to (a) a region of background sky to estimate the noise, (b) two nearby reference stars R1 and R2 whose apparent magnitudes mB are both similar to mB of J1407 out of eclipse, and (c) to a reference star R3 whose mB is comparable to mB of J1407 during the 2007 eclipse (see Fig. 1 and Table 3). If J1407 appears to be significantly dimmer than R1 and R2 and not brighter than R3, it can be assumed that the photographic plate was taken while the ring system was transiting. However, no bona fide plate that showed such a behaviour was found. If J1407 appears to be significantly brighter than R3 and at least as bright as R1 and R2, we confidently can rule out a transit of the ring system. In the other 335 cases, we were not able to confidently either rule out or confirm the photometry of J1407 due to low signal to noise on the plate. This leaves 636 recordings from the DASCH archive for our period searching algorithm.

Table 2

Summary of new photometry extracted from the photographic plates detailed in this paper.

Table 3

Brightness and position of the reference stars used in the visual examination of the photographic plates.

thumbnail Fig. 1

Finding Chart for J1407. Green circled: J1407. Blue: bright reference stars R1 and R2. Red: dim reference star R3. The field of view is approximately 10′× 10′. The plate depicted (“1948.4914_mf37404_00”) stems from the “mf”-series from the DASCH archive and was recorded at 1948-06-28.

2.2 Photographic plates from Bamberg Observatory

The 171 photographic plates from the Bamberg Observatory were taken between 1964 and 1976. We obtained photometry for 129 of them using the APPLAUSEpipeline4. The pipeline did not include photometry for the other 42 plates. All plates were recorded in Bloemfontein, SA, with one of six 10 cm Tessarf∕6 cameras. The errors range between 0.17 and 0.35 mag with a median of 0.23 mag. Twenty of these plates were rejected because of poor photometry, which in total left 109 observations for use in our period folding algorithm.

2.3 Photographic plates from Sonneberg Observatory

There are 103 photographic plates covering J1407 during the years of 1935 and 1938, and between 1952 and 1953 in the photographic plate archive at the Sonneberg Observatory (representing the largest European astronomical photographic archive). Although the Sonneberg Observatory is located in the northern hemisphere, there are a number of southern sky photographic plates and films. These were taken during the expeditions of the former Director of the Observatory, Professor Cuno Hoffmeister, to South Africa. These expeditions took place between November 13, 1934 and February 3, 1938, as well as between August 6, 1952 and July 14, 1953. The Ernostar camera (aperture 135 mm, f = 24 cm, f∕1.8) was used for the observations between 1934 and 1938, while two Tessars (f = 165 mm, f∕3.5) were used for theobservations in 1952 and 1953. All instrumentation was positioned at or near Windhuk, Namibia. The main goal was to monitor selected regions on the southern sky for detection and investigation of variable stars.

The brightness of the star was estimated by an experienced observer (RH) equipped with a magnifying microscope and with the help of nearby constant comparison stars of known magnitudes. We refer to this as the modified Argelander method. Our previous investigations confirmed that this method, if carried out by an experienced observer, can provide accuracy comparable with, and under certain circumstances (e.g. if the position is close to the plate edge and/or star images are affected by coma) even better than that of a measuring machine (Hudec & Wenzel 1976). See Hudec et al. (2013) for more details on the magnitude estimate procedure.

The star was found to be nearly constant and at its bright stage around B = 13.6 on all the investigated plates taking into account the measurement errors. J1407 was close to the magnitude limit on most of the plates so the typical measurement error is ± 0.3 mag. We chose a total of 868 archival recordings of J1407 to use in our period folding algorithm.

thumbnail Fig. 2

Photometry of the data from four CCD sources as well as three photographic plate archives. The offset of the two y-axes between the two separate plots emerges from the different filters with which the observations were taken.

2.4 CCD data from KELT

The Kilodegree Extremely Little Telescope (KELT; Pepper et al. 2007, 2012) survey uses two 42 mm telephoto lenses to search for transiting hot Jupiters around bright (V < 11) stars. KELT-South operates at the South African Astronomical Observatory in Sutherland, South Africa. The telescopes observe the sky with a 10–30 min cadence, depending on their observing strategy. Each telescope has a 26 degree square field of view with a 23 arcsec pixel scale, and can observe stars down to a magnitude V ~ 14 at a photometric error of ~4% (Pepper et al. 2007, 2012)5. KELT-South has observed J1407 a total of 5534 times between 12.03.2010 and 03.02.2015 (see Fig. 2). All these observations show a photometric error around 0.04 mag consistent with the sensitivity of the cameras. They show no sign of a second transit.

2.5 CCD data from AAVSO

In 2012, the American Association of Variable Star Observers (AAVSO)6 started an observing campaign and published an alert to monitor J1407. The campaign is currently ongoing. Since then, amateur astronomershave contributed a total of 4342 observations from various sites with various telescopes showing errors between < 0.05 and 0.3 mag which were derived with standard data reduction techniques. A subset of 99 observations were undertaken either visually or with other filters than Johnson V and will not be used subsequently, leaving 4243 observations for analysis. They do not show any sign of a transit in the light curve.

2.6 CCD data from SuperWASP

The Super Wide Angle Search for Planets (Pollacco et al. 2006; Butters et al. 2010) is a robotic observatory undergoing an all-sky survey of the southern (SuperWASP South) and northern hemisphere (SuperWASP North). Both of these observatories consist of a 2 × 4 array of wide angle cameras on a common mount with overlapping fields of view. The southern telescope in Sutherland started surveying J1407 in 2006 and, apart from the ASAS (All Sky Automated Survey), is the only observer of the transit in 2007. However, since the ASAS observed the star only a small number of times during the transit, we do not use the ASAS data for our algorithm. J1407 lies within the overlap region of three single cameras, the 2007 transit was simultaneously observed by these three different cameras. Mamajek et al. (2012) and van Werkhoven et al. (2014) extensively discussed the photometry from the telescope, including the photometry of the transit. In total, SuperWASP has observed J1407 close to 29 000 times between 05.05.2006 and 09.09.2008. These observations are available in the SuperWASP public archives7.

2.7 CCD data from PROMPT

The Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) network of telescopes was constructed by the University of North Carolina at Chapel Hill (Reichart et al. 2005). Part of the network consists of several telescopes located at the Cerro-Tololo Inter-American Observatory (CTIO)8. Each telescope is configured differently, allowing for users to pick and choose their observations carefully. The telescopes use a submission system that automatically queues observations when requested.

PROMPT-4 and PROMPT-5 at CTIO have been observing J1407 in the Johnson V band since mid-2012. In 2013, observations have been taken with various configurations of 3 s exposures. The FITS images are downloaded and processed using AstroImageJ9. Each image is scanned individually for bad astrometry, hot pixels, and images where clouds moved in or the telescope slewed mid-exposure. Some of the bad astrometry images were salvageable via plate solving to the WCS10. Hot pixels were corrected using a local median smoothing routine in the software. Images where clouds moved in and/or the telescope was slewing were removed. The remaining images are aligned using apertures centred on J1407 and the reference stars. Multi-aperture differential photometry was performed and fluxes were calibrated to the reference star values from Kenworthy et al. (2015). The resulting mean uncertainty of the star’s brightness is 0.018 mag.

3 Analysis

3.1 The period folding algorithm (PFA)

We wrote an algorithm to determine the set of possible orbital periods which are not precluded from a number of observations that bona fide donot show a second transit. An outline of the procedure is as follows: the photometric series of J1407 is represented as m(t) where m is the apparent magnitude of J1407 (possibly at different wavelengths respectively in different filters) at epoch t. It is hypothesized that the dimming of the star in the optical due to a transit of the ring system is sufficiently independent of the observed wavelength. The eclipse in 2007 was detected in Johnson V and the plates used in our analysis were taken in Johnson B. Small dust particles will preferentially scatter and/or absorb blue (shorter wavelength) light, and so the B plates are more sensitive than the V band observations in this case. In the “worst” scenario of the dust consisting of larger particles, the absorption will be approximately wavelength independent. A grey eclipse is therefore a relatively conservative assumption. For further analysis, we will not take into account the photometry of J1407 since the PFA only requires the value of the epoch t of those observations, which with a reasonable certainty do not show an eclipse. To exclude a set of orbital periods of J1407b the whole data set is folded into a test period P such that the beginning of the 2007 transit t0 marks the beginning of the phased light curve, so that a plot of m(t) versus tfold, where tfold = (t − t0) mod P, will show any additional eclipses at period P in the range of tfold from 0 to 52 days (which out of 56 days presents the significant part of the total length transit of 2007).

The 2007 eclipse is approximated as a boxcar function consisting of three transit windows where the photometry was more than 0.5 magnitudes fainter than the out of transit flux levels, as seen in Fig. 3. The central eclipse lasts for approximately 30 days from MJD 54204 to 54234, with the second and third windows lasting 4 days from MJD 54194 to MJD 54298 as well as from MJD 54242 to MJD 54246. For each given trial period P, we count the number of observations within the three transit windows ntransit. If ntransit = 0 for the test period P, it is declared a possible orbital period. If ntransit > 0, the mean magnitude of these points is calculated. If the magnitude of the in transit points is not statistically significant, then we can rule out the test orbital period P, since the photometry of that epoch should exhibit a sign of a transit. We then compute the percentage of possible periods in bins of one year each and normalize the distribution per bin to 100%. We search periods over a range 0 < P < 40 yr with 33 steps per day (corresponding to 12175 steps per year) and plot the resulting binned probabilities in Fig. 4. This number of steps was chosen to be sure that the space between the steps is smaller (about 45 min) than the uncertainty of the recording time of most photographic plates (about an hour).

thumbnail Fig. 3

Lightcurve of the 2007 transit from the SuperWASP-South observatory (black points). The eclipse is approximated with three windows (red line) corresponding to the three major dimming events. The centre of the transit is marked with a dashed blue line at around MJD 54220.
thumbnail Fig. 4

Percentage of possible orbital periods of J1407b inferred from our period folding algorithm with 33 steps per day. Kernel Density Estimator (KDE) of the set of possible orbital periods with a Gaussian Kernel and a bandwidth of 0.04.

3.2 Results

We detect no significant transits of J1407 in the data. The PFA rules out about 90% of the possible periods from 10 to 20 yr. The probability of the trial periods rise to a peak of 20% at 16 yr, falling to almost zero at 21 yr. As can be seen in Fig. 2, a higher-than-average number of plates were recorded in the late 1980s (DASCH), as well as early 1970s (Bamberg), and 1950s (Sonneberg). The folding of these data points thus excludes a great number of possible periods between 18 and 22 yr. The percentage rises again to 25 yr and then stays constant until 40 yr of orbital period at values around 25 to 40%. Since the average density of data points in the Period Folding inversely scales with the test period, we expect the resulting percentage of possible periods to constantly rise for test periods of 30 yr and more. We cannot confidently rule out any period above 25 yr.Since very few plates have been taken before 1900 – 107 yr before the 2007 transit – this method effectively cannot rule out any periods above 100 yr. We expect the next transit no earlier than Spring 2018 (corresponding to an orbital period of 11 yr) and most plausibly between Spring 2021 (orbital period of 14 yr) and Spring 2024 (orbital period of 17 yr).

Analysis using histogram plots is susceptible to the precise placement of the bin edges. To mitigate this sensitivity, a method called Kernel Density Estimation (KDE) uses a smooth, continuous function (the kernel) to convolve with the data and reduce sensitivity to bin edge placement. We apply KDE to the data and plot the results in Fig. 4. We used a Gaussian Kernel and a standard Scott and Silverman bandwidth selection for the subsets of periods below 20 yr and a bandwidth of 0.04 for the whole data set. The later value was used to have a meaningful depth of details over the entire course of the plot. The results strongly constrain the orbital period to a value between 11 and 18 yr and values above 22 yr. As the probability density rises from 11 yr onward, more probable periods range between about 14 and 17 yr with the most probable period around 16.5/17 yr. The peaks of the density function at or close to integer year periods have the following origin. Since J1407 is not a circumpolar star, we lack photometric data for more than 50% of the year. For integer test periods, a number of these gaps in the data thus have about the same phase. If these gaps now happen to overlap with the test windows of the PFA, it is likely that the PFA yields a higher density of possible periods, hence the peaks seen at integer year spacings. If our photometry was distributed homogeneously across the course of the year, this feature would be absent. This phenomena is also present for periods higher than 20 yr as can be seen in Fig. 4. The KDE used is the class scipy.stats.gaussian_kde implemented with scipy.

4 Discussion

4.1 Circular orbits

For a given orbital period P and a mass of the secondary companion mb, constraints of the size of its Hill sphere can be derived. Together with the transverse velocity of the companion and the duration of the 2007 eclipse, we calculate the size of the putative ring system in units of the Hill sphere diameter, ξ. Given Keplerian circular orbits for J1407b, any period greater than 14 to 15 yr is improbable considering they cannot explain the high orbital velocity derived from the rate of change of the light curve (Mamajek et al. 2012; Kenworthy et al. 2015). The coplanarity of a giant ring system with a significant obliquity was examined in Zanazzi & Lai (2017), and shown that even for circular orbits, it is impossible to keep ring systems aligned with the Laplacian plane of the planet – at a large enough radius, the obliquity of the rings will decrease to zero. It therefore seems highly unlikely that J1407b is on a circular and bound orbit.

4.2 Elliptical orbits

The rapid variations in the 2007 light curve can be explained by a highly elliptical orbit with the transit occurring close to or during periastron. Since a longer orbital period infers a higher ellipticity, the minimum orbital period of 11 yr only allows for highly elliptical solutions of 0.72 < e < 0.78 (Kenworthy et al. 2015). van Werkhoven et al. (2014) have argued that longer periods are unlikely due to the disruption of the ring system during periastron passage, if the constraint of the large transverse velocity derived from the light curve is kept. Combined with the general constraints on the orbital period from this paper, these dynamical arguments render orbital periods of more than 18 yr very unlikely. If the orbital period is from 11 to 14 yr, there is the possibility that the companion can have a mass up to 80 Jupiter masses (Kenworthy et al. 2015), although this has a low probability as seen in Fig. 4. If the orbital period is from 14 to 18 yr, we can then constrain the mass of the ringed companion to be 5 to 20 Jupiter masses with ξ < 1 for the majority of the orbit.

However, at periastron passage, the instantaneous ξ is greater than one causing rings to spill out of the Hill sphere. Rieder & Kenworthy (2016) explored the full parameter space of secondary companion mass, ellipticity and periodicity. The conclusion was that prograde orbiting rings were not stable enough through periastron passage to not lose a significant fraction of the outer rings, but that retrograde orbits for the ring particles allow for significantly larger rings out to 60% of the Hill sphere radius and significant survival of the outer rings through periastron. Adding retrograde orbits further deepens the challenge of explaining the nature and stability of the ring system seen towards J1407.

5 Conclusions

In this paper we have presented photometry from 868 photographic plates from 1890 to 1990, and have combined them with more recent epoch photometry to constrain the orbital period of J1407b. We detect no statistically significant dimming of the star in all the photographic plates. We can rule out 90% of all periods between 10 and 20 yr, falling rapidly to a minimum from 18 to 22 yr. Elliptical orbits with periastron passage during May 2007 allow a range of possible masses from 5 to 20 MJup and infer an ellipticity of about 0.75. In this case, the rings would fill 70–100% of the Hill sphere of companion, and exceed the shrinking hill sphere during periastron by at least a factor of two. Although the range of orbital periods has been reduced, the lack of the detection of another eclipse means that we still cannot confirm that the eclipse event in 2007 was due to an extended object on a bound orbit around J1407.

Two possible future observations that can resolve the nature of the J1407 eclipse include continued photometric monitoring for the next eclipse, and direct detection of radiation from the rings themselves, either at sub-mm wavelengths with ALMA, or in optical wavelengths through the polarization of reflected light from the star.

Acknowledgements

We express our deepest gratitude to Alison Doane, former Curator of the Harvard Astronomical Plate collection, who sadly passed away during the course of this research. Her expansive knowledge of the history of the Harvard Plates and the Harvard women computers who studied them, and her gracious patience in instructing us on how best to use the plates was instrumental to this research. Her quick action during the flood of Harvard College Observatoryin January 2016 was vital in saving the photographic plates and preserving this unique historical resource for many future generations. Part of this work was supported by the DAAD programme allowing RTM to carry out the firstpart of this research in Leiden Observatory during the Summer of 2016. M.A.K. thanks DASCH for allowing access to their archives. The DASCH project at Harvard is grateful for partial support from NSF grants AST-0407380, AST-0909073, and AST-1313370. Funding for APPLAUSE has been provided by DFG (German Research Foundation, Grant), the Leibniz Institute for Astrophysical Research (AIP), the Dr. Remeis Sternwarte Bamberg (University Nuernberg/Erlangen), the Hamburger Sternwarte (University of Hamburg) and Tartu Observatory. R.H. thanksSonneberg Observatory for access to its plate archive and acknowledges GACR grant 13-33324S. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This research made use of Astropy, a community-developed core Pythonpackage for Astronomy (Astropy Collaboration 2013). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. S.N.M. is a U.S. Department of Defense SMART scholar sponsored by the U.S. Navy through SSC-LANT.This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. E.E.M. and S.N.M. acknowledge support from the NASA NExSS programme. Part of this research was conducted at Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We thank the referee for their time and constructive comments that have improved this paper.

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1

V1400 Cen was assigned by Samus et al. (2017) in the General Catalogue of Variable Stars version 5.1.

2

This is in good agreement with earlier kinematic distance estimates (128 ± 13 pc; Mamajek et al. 2012), and the mean distance to the UCL subgroup of Sco–Cen (142 pc; de Zeeuw et al. 1999). A calculation with the BANYAN Σ tool (Gagné et al. 2018) using these new parameters yielded a near-certain probability of UCL membership of 96.1%.

3

Their website with the access to their photometry can be found under http://dasch.rc.fas.harvard.edu/lightcurve.php.

All Tables

Table 1

Stellar parameters of J1407 from (1): Zacharias et al. (2010), (2): Mamajek et al. (2012), newly calculated with data from (3): the Gaia Data Release 2, and from (4): Kenworthy et al. (2015).

In the text
Table 2

Summary of new photometry extracted from the photographic plates detailed in this paper.

In the text
Table 3

Brightness and position of the reference stars used in the visual examination of the photographic plates.

In the text

All Figures

thumbnail Fig. 1

Finding Chart for J1407. Green circled: J1407. Blue: bright reference stars R1 and R2. Red: dim reference star R3. The field of view is approximately 10′× 10′. The plate depicted (“1948.4914_mf37404_00”) stems from the “mf”-series from the DASCH archive and was recorded at 1948-06-28.
In the text
thumbnail Fig. 2

Photometry of the data from four CCD sources as well as three photographic plate archives. The offset of the two y-axes between the two separate plots emerges from the different filters with which the observations were taken.
In the text
thumbnail Fig. 3

Lightcurve of the 2007 transit from the SuperWASP-South observatory (black points). The eclipse is approximated with three windows (red line) corresponding to the three major dimming events. The centre of the transit is marked with a dashed blue line at around MJD 54220.
In the text
thumbnail Fig. 4

Percentage of possible orbital periods of J1407b inferred from our period folding algorithm with 33 steps per day. Kernel Density Estimator (KDE) of the set of possible orbital periods with a Gaussian Kernel and a bandwidth of 0.04.
In the text

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