Astronomy:HD 21749
 HD 21749 – star in the constellation Reticulum | |
| Observation data Equinox J2000.0]] (ICRS) | |
|---|---|
| Constellation | Reticulum |
| Right ascension | 03h 26m 59.22s[1] |
| Declination | −63° 29′ 56.9″[1] |
| Apparent magnitude (V) | 8.143[2] |
| Characteristics | |
| Spectral type | K4.5V[3] |
| U−B color index | 1.144[2] |
| B−V color index | 1.159[2] |
| Astrometry | |
| Radial velocity (Rv) | 59.32±0.12[1] km/s |
| Proper motion (μ) | RA: 355.20[1] mas/yr Dec.: -247.39[1] mas/yr |
| Parallax (π) | 61.2271 ± 0.0150[1] mas |
| Distance | 53.27 ± 0.01 ly (16.333 ± 0.004 pc) |
| Absolute magnitude (MV) | 6.95[4] |
| Details[5] | |
| Mass | 0.73±0.07 M☉ |
| Radius | 0.695±0.030 R☉ |
| Luminosity | 0.20597±0.00016 L☉ |
| Surface gravity (log g) | 4.613+0.052 −0.061 cgs |
| Temperature | 4640±100 K |
| Metallicity [Fe/H] | 0.003±0.060 dex |
| Rotation | 34.1+2.4 −2.7 d[6] |
| Rotational velocity (v sin i) | 1.04 km/s |
| Age | 3.8±3.7 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
| Extrasolar Planets Encyclopaedia | 21749 data |
HD 21749 is an orange main-sequence star in the constellation Reticulum. It has an apparent visual magnitude of 8.143, which means it is too dim to be seen with the naked eye. From parallax measurements by the Gaia spacecraft, it is located 53 ly (16 pc) from Earth.
In 2019, it was discovered that the star has two exoplanets: a possibly rocky, hot sub-Neptune-sized exoplanet named HD 21749 b; and an Earth-sized exoplanet named HD 21749 c. These exoplanets were discovered by the TESS spacecraft.
Stellar characteristics
HD 21749 is a K-type main sequence star (orange dwarf) with a spectral type of K4.5V,[3] indicating it is smaller and cooler than the Sun. It is estimated to have a mass of 0.73 M☉, a radius of 0.70 R☉, and a luminosity of 0.20 L☉. Its effective temperature is 4,640 K, which gives the star an orange color typical of K-type stars. Its metallicity—the proportion of elements other than hydrogen and helium—is approximately equal to the Sun's.[5]
This star is moderately active, as shown by its spectral activity indicators and photometric data. These measurements indicate a rotation period of around 30 to 40 days, with a most likely value of 34 days. Stellar activity also creates radial velocity variations, which complicates the measurement of the mass of the planets in the system.[6]
A companion to HD 21749 is listed in double star catalogues, a 9th-magnitude star separated by 22″ in 2015, although decreasing rapidly due to the high proper motion of the primary.[8] The companion is much more distant than the primary and the two are unrelated except being coincidentally in the same line of sight.[9]
Planetary system
In January 2019, the discovery of an exoplanet around HD 21749 was published. The planet was identified from a single transit event detected by the TESS spacecraft, using data from the first two observation sectors of the mission. Since a single transit is insufficient to determine the orbit of a planet, astronomers used archival radial velocity data from the HARPS spectrograph to detect the planet's signal, which allowed the determination of its orbital period and mass.[10] In April 2019, with two additional months of data from the TESS spacecraft, the orbital period of the planet was confirmed with the observation of new transits, and a second planet was discovered.[5]
HD 21749 c
The inner planet, HD 21749 c (the second in order of discovery), is orbiting the star at a distance of 0.08 AU with a period of just 7.8 days. A terrestrial planet, it has a radius of 1.1 R⊕ and was the first Earth-sized planet found by TESS.[6][5] Its mass is too low to be calculated with current radial velocity data, with an upper limit of 3.5 M⊕; a probabilistic model estimates it is most likely between 1 and 2 M⊕.[6]
HD 21749 b
The outer planet, HD 21749 b, orbits the star at a distance of 0.21 AU with a period of 35.6 days. With a mass of 20 M⊕ and a radius of 2.9 R⊕, it is similar to Neptune but much denser. Its density of 4.7 g/cm³ suggests it is composed of a substantial rocky core, with a radius of approximately 2.1 R⊕, plus a relatively thick gaseous layer.[6]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| c | <3.5 M⊕ | 0.076±0.008 | 7.7902+0.0004 −0.0006 |
0 | 89.44+0.36 −0.52° |
1.13+0.11 −0.10 R⊕ |
| b | 20.0±2.7 M⊕ | 0.209+0.022 −0.021 |
35.6133+0.0005 −0.0006 |
0.164+0.062 −0.058 |
89.40+0.07 −0.08° |
2.86+0.21 −0.20 R⊕ |
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940 Gaia DR3 record for this source at VizieR.
- ↑ 2.0 2.1 2.2 Koen, C. et al. (April 21, 2010), "UBV(RI)CJHK observations of Hipparcos-selected nearby stars", Monthly Notices of the Royal Astronomical Society 403 (4): 1949–1968, doi:10.1111/j.1365-2966.2009.16182.x, Bibcode: 2010MNRAS.403.1949K
- ↑ 3.0 3.1 Gray, R. O. et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, doi:10.1086/504637, Bibcode: 2006AJ....132..161G
- ↑ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters 38 (5): 331, doi:10.1134/S1063773712050015, Bibcode: 2012AstL...38..331A
- ↑ 5.0 5.1 5.2 5.3 Dragomir, Diana et al. (2019), "TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune", The Astrophysical Journal 875 (2): L7, doi:10.3847/2041-8213/ab12ed, Bibcode: 2019ApJ...875L...7D
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Gan, Tianjun; Wang, Sharon Xuesong; Teske, Johanna K.; Mao, Shude; Howard, Ward S.; Law, Nicholas M.; Batalha, Natasha E.; Vanderburg, Andrew et al. (2021), "Revisiting the HD 21749 planetary system with stellar activity modelling", Monthly Notices of the Royal Astronomical Society 501 (4): 6042, doi:10.1093/mnras/staa3886, Bibcode: 2021MNRAS.501.6042G
- ↑ "HD 21749". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+21749.
- ↑ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001), "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog", The Astronomical Journal 122 (6): 3466, doi:10.1086/323920, Bibcode: 2001AJ....122.3466M
- ↑ Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940 Gaia DR3 record for this source at VizieR.
- ↑ Trifonov, Trifon; Rybizki, Jan; Kürster, Martin (2019), "TESS exoplanet candidates validated with HARPS archival data. A massive Neptune around GJ 143 and two Neptunes around HD 23472", Astronomy and Astrophysics 622: 622, doi:10.1051/0004-6361/201834817, Bibcode: 2019A&A...622L...7T
External links
- Staff (2019). "Stellar Overview Page - HD 21749". NASA Exoplanet Archive. https://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=HD%2021749. Retrieved 8 January 2019.
- Overbye, Dennis (7 January 2019). "Another Day, Another Exoplanet: NASA's TESS Keeps Counting More". The New York Times. https://www.nytimes.com/2019/01/07/science/exoplanets-tess-nasa.html. Retrieved 8 January 2019.
- Massachusetts Institute of Technology (16 April 2019). "TESS discovers its first Earth-sized planet - Orbiting a nearby star, the new planet is the smallest identified so far by the TESS mission". EurekAlert!. https://www.eurekalert.org/pub_releases/2019-04/miot-tdi041619.php. Retrieved 16 April 2019.
Coordinates: 
03h 26m 59.22s, −63° 29′ 56.9″
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