synonymTransit method(Transit method) generally refers to planetary transit method
Planetary transit method is an observation method that calculates the orbit and mass parameters of planets by analyzing the brightness changes of stars when the transit phenomenon occurs.
Its observation principle is that during the transit,fixed starThe brightness of the star is weakened due to the obscuration of the planet in front of it, and this phenomenon is periodic, so it can be detected that there are planets around the star.This method is the most widely used method for observing exoplanets by 2015.
Astronomers have found manysolar systemouter planet.When exoplanets orbit their star to the side of the star facing the earth“Venus transit”Similar phenomenon is called "transit".When transit occurs, the light of the star is weakened due to occlusion.Astronomers can determine the orbital inclination of exoplanets and then determine their mass through the brightness change of stars.The method of searching for extraterrestrial planets by observing transit stars is called planetary transit method.[1]
Fundamentals
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The basic principle of the transit method is that for those alien planets whose orbital plane of revolution is very close to the direction of line of sight, the planets may pass in front of the parent star, as if it happened in the solar systemMercury transitOr Venus transit, which is called planetary transit in astronomy.During the transit, the brightness of the star will be weakened by being obscured by the planet in front, and this phenomenon is periodic, so it can be detected that there are planets around the star.This method can also work with small telescopes, but it is applicable to fewer objects.Of course, the degree of star brightness reduction due to the transit phenomenon is very small. When the transit occurs, a Jupiter sized planet will reduce the brightness of the parent star by about 1%, while the corresponding number for the Earth sized planet is only 0.01%.It can be seen that in order to discover alien planets by this way, we must have high photometric accuracy.[2]
Observation mode
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Collect data
First, determine the target source for observation, select the target planet for convenient observation according to the location of the observatory telescope used, and select the starApparent magnitudePlanets with greater interference are observed.
Next, determine the transit time.Determine the prediction time of the transit event according to the recorded data provided by the international transit observation website, arrange the instrument to start observation and use the V filter 0.5~1.5 hours before the prediction time, determine the appropriate exposure time according to the brightness of the source, and finally stop the observation 0.5~1.5~h after the end of the transit event, to prevent the observation source transit process from not being photographed completely due to the inaccurate prediction time.
Finally, the observation data are obtained according to the shooting data.[3]
Error correction
After the transit observation data is obtained, the pre-processing of the data is carried out. Due to the thermal electronic noise of the CCD itself in the working processThe sensitivity difference between CCD pixels and other factors introduce additional effects, so the CCD image obtained from observation does not fully reflect the situation of the captured sky area. In order to eliminate these additional effects, first of all, the image must be preprocessed, including zero field correction, dark field correction and flat field correction. Normally, multiple zero field, dark field and flat field images are taken during normal observation,Before correction, use zerocombine, darkcombine and flatcombine under imred/cedred package to merge multiple images of zero field, dark field and flat field, and then use ccdproc command to correct the target image.[4]
data processing
After these preprocessing stages, the MaxiM DL software is used to perform poor photometry for the star to be measured, the surrounding comparison star and the calibration star. Generally, two reference stars and one calibration star with similar brightness to the main star in the field of view are selected.The positions and stars of reference stars and check stars are from SIMBAD and USNO online catalogs.In the process of poor metering, images that are unavailable due to weather or observation reasons should be eliminated according to the actual situation.The steps of applying MaxIM DL to poor metering are as follows: use MaxIM DL to open the preprocessed image, use the photometric command to select the target star, reference star and check star, manually write the magnitude of the reference star found and set the aperture value for metering.After setting and metering, the magnitude values of the target star, reference star and calibration star can be obtained. Finally, the light variation curves of the star to be measured and the calibration star can be obtained for analyzing the transit events and estimating the relative metering accuracy, respectively.[5]
Photorefractive curve
Fig. 1 Light variation curve
Using the observed transit events and the aperture difference photometry method, the light variation curve of the star to be measured is obtained.
The horizontal axis is the Julian day, and the vertical axis is the magnitude (my) of the V band.The solid dot represents the main star (obj) of the exoplanet to be measured, and the hollow triangle represents the check star (chk) (in order to facilitate comparison, the magnitude value of the check star is translated accordingly).Figure 1 shows the start time, end time and middle (JDmid) time of the transit obtained by fitting the measured light variation curve, and also gives the relative accuracy value of poor light measurement.[6]
Parameter estimation
The radius of the planet can be accurately measured by the transit method, and then the density of the planet can be estimated by combining the planet mass obtained by the radial velocity method.The data statistics after multiple transit observations of the same planet will help to estimate its parameters more accurately.[7]
Model fitting
After obtaining the transit light curve, the TRECSA website conducts data model fitting for the light curve, and Poddany et al. give the model fitting method as follows: for the original observation data of the uploaded light curve, it is assumed that the observation data is included in the ti(i=1, 2, 3...) Relative magnitude m (ti）, simulate the data using the following formula:
Where F (z, P, C1) is the relative flow value of the host star (star), which is caused by the planetary transit phenomenon. P=Rp/R * is the radius ratio of the planet and its host star, R * is the radius of the star (the theoretical value is given by the TRECSA website), RP is the radius of the planet, and Rp is far less than R * (P ≤ 0.2)
The edge darkness of the star is fitted by the linear equation with coefficient C1. The estimated distance between the planet and the star is z. Set to as the intermediate time of the transit;D is the duration of the whole transit phenomenon;The planetary motion track is simulated as a straight line passing through the stellar disk, which is represented by the parameter b=acosI/R *, where a is the semi major diameter of the planetary orbit, and I is the orbital inclination;The function z [ti, to. D, b] at each time of ti can be calculated by the above settings. F (z,P. The calculation of c1). (1) The variable A in the formula describes the zero drift of the magnitude, and the variables B and c describe the systematic change trend of the data. The calculation of the first and second terms of the average observation time tmaan=∑ ti/N is used to reduce the numerical error. There is no explicit correction of the atmospheric mass curvature, because the general quadratic polynomial is sufficient in most cases[8]
Parameter estimation
The main objective of extrasolar planet exploration and research by transit method is to obtain the transit intermediate time to, duration D, transit depth △ F. These important parameters can be determined by (1)In the formula, the model is fitted. The planetary geometric parameters that can be more accurately estimated by the transit method are the planetary radius and orbital inclination. The following methods are given by the TRECSA website to calculate the parameters. The planetary radius is calculated by the following formula:
Where △ F is the transit depth, F * is the total stellar flux at non transit time, Ft is the stellar flux at transit time, and I * is the stellar intensity
Planetary radius
The orbital inclination can be obtained from the total transit time tz of the planet and the revolution period P of the planet (the theoretical value is given by the TRECSA website), and the specific formula is
Rail inclination
follow-up work
1. Establish a systematic observation, analysis and processing scheme based on the observed data, and promote it to more transit detection.
2. Use the improved scheme to observe exoplanets without transit data, and summarize their motion laws and important parameters.
3. Use this scheme to explore unknown exoplanets and analyze their physical laws.[8]
Main role
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The transit method can determine the orbital inclination of extrasolar planets according to the periodicity of the brightness changes of stars, so as to further determine the mass of planets.The transit method can also understand the atmospheric structure of planets. When a planet passes its parent star, the light from the parent star will pass through the outermost atmosphere of the planet.As long as the spectrum of the parent star is carefully analyzed, the atmospheric composition of the planet can be known.By subtracting the spectrum of the second eclipse (that is, the planet is covered by its parent star) from the spectrum before and after the second eclipse, we can directly get the spectral properties of the planet, so as to know the temperature of the planet, and even detect the formation of clouds on the planet.[9]
research meaning
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By measuring the brightness of stars, we can detect the existence of planets and get more accurate planet information.In addition to detecting and searching for newExtrasolar planetsUsing the transit observation data, we can also obtain some planetary physical parameters, such as radius and density, which are difficult to obtain by other detection methods (such as the radial velocity method, etc.), which plays an irreplaceable role in comprehensively and deeply understanding the physical properties of the planet.
In addition, even for extrasolar planetary systems that have been observed before, new and more accurate transit observation data are of great significance for obtaining accurate planetary physical parameters, and even for finding the existence of new extrasolar planets by using the change of transit time and duration (i.e. transit timing variation method).[10]
research status
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international
The transit method will be used in the convection rotation and planetary transits of the European Space Agency (ESA) and the Kepler Mission of the National Aeronautics and Space Administration (NASA).COROT can detect planets slightly larger than the Earth, while Kepler Space Telescope is more capable of detecting planets smaller than the Earth.expectKepler space telescope It is also capable of detecting the reflection of small orbits and large planets, but it is not enough to form an image;For example, the phase of the moon, these reflections will increase or decrease with time, and the analysis of these data can even show the material distribution in its atmosphere.Kepler can find more undiscovered exoplanets through this method.NASA plans to launch the space interference mission in 2014 to use astrometry to find Earth like planets among nearby stars.
European Space AgencyDarwin Plan(Darwin) probe and NASA'sterrestrial planetTPF (Territorial Planet Finder) will try to take pictures of exoplanets directly.The New Worlds Imager proposed in 2013 has more shading equipment to block the light of stars, allowing astronomers to directly observe dim exoplanets.[11]
domestic
In 2011, the National Astronomical Observatory of the Chinese Academy of SciencesLiu YujuanAnd others used Xinglong Observation Station and JapanOkayamaThe telescope of the Astrophysical Observatory (OAO) found an exoplanet by transit method, and gave important parameters.
Qian Shengbang, a researcher of Yunnan Observatory, and others found exoplanets orbiting white dwarf binaries using the 60cm, 1m and 2.4m telescopes of Yunnan Observatory.
Wang Xiaobin of Yunnan Observatory and others used the 1m telescope of Yunnan Observatory to conduct photometric observation and data processing analysis on three observation targets of the SuperWASP project, and obtained some basic parameters of target stars.
