Object 65: HD 106252
Podcast release date: 24 January 2022
Right ascension: 12:13:29.5
HD 106252 is a Sun-like star at a distance of 103.7 light years (38.1 pc) in the constellation Virgo [1, 2]. It's not just a Sun-like star in that it fuses hydrogen into helium in its core. It's actually almost identical to the Sun. It's mass matches the mass of the Sun to within a couple of percent, and it's surface temperature is close to that of the Sun, so it has the same yellow color . It could cosplay as the Sun at comic book conventions.
If you do a Google search on HD 106252, you won't see any photos of the star at the San Diego Comic-Con from 2015, but you will see that the star system is known on the internet for having an exoplanet, which is uncreatively named HD 106252b. The discovery of the exoplanet was published in a paper in 2002 led by by Debra Fischer . She had been working with a research group that had been using the Keck and Lick observatories to carefully observe hundreds of stars for exoplanets. The group identified the presence of exoplanets by how their gravity caused slight changes in the motions of the stars that they orbited.
HD 106252b was identified to be about 7 times the mass of Jupiter . The exoplanet orbits the star at an average distance of 2.4 astronomical units (AU) . For context, the distance from the Earth to the Sun is 1 AU, the distance from the Sun to Mars is 1.5 AU, and the distance from the Sun to Jupiter is 5.2 AU, so if HD 106252b was in our Solar System, it would orbit within the asteroid belt. However, HD 106252b is on a very elongated, elliptical orbit, so it can get as close as 1 AU or as far as 3.8 AU from its star .
However, 80 other stars with exoplanets had been discovered before the exoplanet in orbit around HD 106252, so it was not that exciting when it was discovered . HD 106252b also didn't stand out in any specific way when it was discovered and doesn't stand out now. For example, it isn't Earth-like, its orbit isn't all that unusual, and no interesting measurements have been made of the exoplanet's atmosphere, so no one has really paid much attention to it after its discovery. To put this into context, think of all of the great musicians that have worked with Motown Records. The most famous musicians would include Michael Jackson (along with the Jackson 5) and Diana Ross (along with the Supremes) and Smokey Robinson and Marvin Gaye and Stevie Wonder and the Temptations and the Four Tops and so on and so forth. You can think of HD 106252b as being as famous as musicians who rank much lower in the list of famous Motown artists, like Gino Parks, the Velvelettes, and Bobby Taylor and the Vancouvers.
So, while HD 106252's exoplanet has sort of faded into obscurity, astronomers have become very interested in the star for very different reasons. It turned out to be very useful as a calibration source.
In the early 2000s, astronomers working with the Spitzer Space Telescope, which performed observations in the infrared, wanted to develop a better way of calibrating their telescope. The measurements made by many telescope detectors, including the infrared detectors on the Spitzer Space Telescope, are often in some sort of electronic units that do not necessarily indicate exactly how many photons are coming from the objects that astronomers are looking at. To calibrate these instruments, astronomers point their telescopes at objects where the total energy output is very well known. When they do this, they will make measurements with their detectors in electronic units that they know correspond to a specific amount of observed energy, and they can use these data to create conversion factors between the electronic and astronomical units that can then be applied to observations of objects with unknown brightnesses.
George Rieke, who was the lead scientist for one of the instruments on the Spitzer Space Telescope and who is also well known for wearing plaid, was one of the people who carefully investigated this topic. He demonstrated that, because the spectrum of the Sun in the infrared had already been carefully measured at infrared wavelengths, it was possible to calculate the infrared brightnesses of other stars that looked very similar to the Sun, and it was possible to use those stars as infrared calibration sources .
However, it's not possible to use just any star that looks like the Sun as an infrared calibration source. First of all, the stars have to be bright enough that they are easy to detect with infrared telescopes but not so bright that they effectively blind the instruments (or, in other words, infrared telescopes can't just point at the Sun as well as a few other really bright Sun-like stars in the sky). Second, the stars have to already have reliable brightness measurements either in or near the visible part of the electromagnetic spectrum. Third, the stars cannot show any signs of being surrounded by any type of dust disk, which would produce its own infrared emission that could not be easily estimated using models of the Sun's spectrum or any other models .
Now, I had mentioned that HD 106252 looked so similar to the Sun that it could be cast as Antipholus of Syracuse in Shakespeare's Comedy of Errors. Because it is relatively close to the Earth, it's bright but not too bright, and its brightness in and near the visible part of the electromagnetic spectrum has been fairly accurately measured. Also, even though it has an exoplanet, it does not have a dust disk, so virtually all of the infrared emission from the star system comes from the star itself. So, HD 106252 was selected for use as an infrared calibration source for the Spitzer Space Telescope [4, 5].
However, the Spitzer Space Telescope is not the only infrared telescope to ever operate in space. Other telescopes could also potentially use HD 106252 as a calibration source, and one of those telescopes is the James Webb Space Telescope. While the James Webb is often described as a replacement for the Hubble Space Telescope, the James Webb Space Telescope was actually designed to observe infrared light with wavelengths far longer than what Hubble could observe. This means that the telescope also needs to observe infrared calibration sources so that the astronomers know how much light they are observing from the objects that the telescope looks at. So, the James Webb will be looking at many of the same calibration sources used by the Spitzer Space Telescope, and that includes HD 106252 [6, 7].
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533
 Fischer, Debra A. et al., Planetary Companions to HD 136118, HD 50554, and HD 106252, 2002, Publications of the Astronomical Society of the Pacific, 114, 529
 Rieke, G. H. et al., Absolute Physical Calibration in the Infrared, 2008, Astronomical Journal, 135, 2245
 Engelbracht, C. W. et al., Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 micron Calibration, 2007, Publications of the Astronomical Society of the Pacific, 119, 994
 Gordon, K., and Bohlin, R., JWST Absolute Flux Calibration II: Expanded Sample of Primary Calibrators, 2011
 Krick, Jessica E. et al., Spitzer IRAC Photometry of JWST Calibration Stars, 2021, Astronomical Journal, 161, 177