Dr. Gerold Holtkamp, ​​December 5, 2024

The measurement of the transit light curve of the exoplanet TOI-2570 b, which was only discovered in 2022, is described. The brightness of the parent star, which is located in a very dense field of stars, was determined in 148 images in relation to comparison stars. The result agrees very well with the data known so far.

Introduction

The constellation Fuhrmann is already completely above the horizon from 5 p.m. in December. Its bright star, Capella, cannot be overlooked even in a city like Osnabrück. However, the 12.4 mag bright star TOI-2570 in Fuhrmann is only visible in the telescope. It is close to the open star cluster M 37, but only apparently, because at a distance of 4,400 light-years, M37 is very far behind TOI-2570, which is 1,180 light-years away [1]. The dense star field of the surrounding Milky Way makes it not exactly easy to find. But with the support of so-called platesolving (AstroArt software), a clear identification was possible [2].

It was only proven in 2022 that TOI-2570 has at least one planet of its own. This was made possible with the help of the TESS (Transiting Exoplanet Survey Satellite) probe, which was launched in April 2018. TESS searched and continues to search for stars where a planet passes in front of the latter exactly in the line of sight between the probe and the star. A regularly recurring decrease in brightness then indicates a possible planet. As a so-called TESS Object of Interest (TOI), TOI-2570 b (b denotes the planet) was examined using additional methods (photometry, high-resolution photography and spectroscopy) and its planetary nature was confirmed [3].

Measurement

As with TESS, amateur astronomers can also use the transit method to detect exoplanets. The transit method has already been explained elsewhere [4]. The earth's atmosphere and - if you live in the city - the many disturbing light sources make the measurement difficult, but do not make it impossible. The interested reader should judge this for himself below.

The equipment used was as follows:
Telescope: Skywatcher Newton with 250 mm aperture and 1,200 mm focal length
Camera: QHY268M, -10° C, Offset 20, Gain 50
Filter: Antlia Luminanz
Mount: AZ EQ6
Autoguiding: Off-Axis-Guider
Controls: AstroArt
Evaluation: HOPS 3.3.0

The HOPS software is provided by ESA's Exoclock project. This project was launched to organize the measurement of as many transit light curves as possible for selected exoplanets. It serves to prepare for the Ariel mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey), which will record thousands of exoplanet transits and their spectra from 2029 in order to study the chemical composition of the atmospheres [5]. This requires the best possible knowledge of the transit times and the brightness curves of the parent stars. This is exactly where the small and medium-sized telescopes of non-professional astronomers come into play.

The HOPS software initially provides the image correction of the individual recordings using flats, darks, flat darks and biases. This is followed by the photometry of the stars. The measuring points and the compensation curve with trend correction are then created from this by comparing the brightness of the target star and the brightness of the comparison stars. The measured values ​​can then be derived from this.

On November 30, 2024, the exoplanet TOI-2570 b should pass in front of its parent star at 9:49 p.m. CET and end the transit at 12:45 a.m. CET. In order to preserve the brightness of the undimmed star, the measurement was started at 8:49 p.m. CET and ended at 1:49 a.m. CET. At the beginning of the measurement the star was 36° high and at the end it was 70°. The moon wasn't in the sky. There was a slight frost that night. There was no wind. However, there was a lot of moisture in the air, which was evident in the heavy hoarfrost on the telescope in the morning.

148 individual measurements of 120 s each were carried out. To adjust the recordings, 10 recordings each of flats, flat darks, darks and biases were recorded. For the relative light measurement, four comparison stars were used that have a similar brightness to the target star TOI-2570.

Results

The HOPS software directly calculates two values. The ratio of undisturbed to obscured flux is equal to the ratio of planet radius to star radius: Rp/Rs = 0.1155 ± 0.0046 (Exoclock 0.1142 ± 0.0012). If the star radius is known, which can be determined using other methods, one can directly calculate the radius of the exoplanet TOI-2570 b: Rp = 88,470 +/- 3,523 km. My measurement thus confirms that TOI-2570 b is only about 11% larger than Jupiter (69,911 km) in our solar system. (For the calculation of Rp/Rs see [6])

Determining the exact timing of the transit is an important task of the Exoclock project in preparation for ESA's Ariel mission. I measured: O-C (observed minus calculated transit center) = 3.33 ± 1.87 minutes

Discussion

The measurement is most probably influenced by the widespread Christmas lighting. The “unrest” at the beginning of the lightcurve, when the star was still at an altitude of 35°, is attributed to the illumination of the neighborhood in the observation direction. The “dent” before the middle of the transit may have come from the disturbing influence of a construction crane boom illuminated from below, which almost protruded into the field of vision at the corresponding time.

As can be seen from Fig. 4, there are only a few measurements. Even among professional astronomers there are only a few (see publication). It's nice when, as an amateur astronomer, you can get up close and contribute your own measurements. It is still particularly exciting to determine the radius of a planet at such a huge distance, even though you only have one point of light - the star - available!

appendix

The system data known to date: [7]

The star TOI-2570
Spectral type: G variable
Apparent brightness: 12.4 mag (V)
Distance: 1,180 light years
Surface temperature: 5,771K
Mass: 1.06 x mass of the Sun
Radius: 1.1 x Radius Sun
Age: 4.4 +3.3/-2.6 billion years
Constellation: Auriga

The Exoplanet: TOI-2570 b
Discovery: 2022
Distance to parent star: 3.38 million km
Orbit period: 2.99 days
Orbit (semimajor axis): 6.2 million km
Surface temperature: 1,430 K
Mass: 1.9 times the mass of Jupiter
Radius: 90,884 km (radius of Jupiter 69,911 km)

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Sources

[1] https://de.wikipedia.org/wiki/Messier_37
[2] https://en.wikipedia.org/wiki/Astrometric_solving
[3] The TESS Grand Unified Hot Jupiter Survey. I. Ten TESS Planets, Samuel W. Yee et al 2022 AJ 164 70
[4] https://kosmos-os.de/messung-der-transitkurve-des-exoplaneten-tres-3-b-am-27-5-2023/
[5] https://www.exoclock.space/ and https://arielmission.space/
[6] https://kosmos-os.de/messung-der-transitlichtkurven-der-exoplaneten-wasp-84b-und-kps-1b-am-7-und-8-maerz-2024/
[7] https://iopscience.iop.org/article/10.3847/1538-3881/ac73ff