Draft:7 Sextantis
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7 Sextantis is an early A-type star in the constellation Sextans. The star has a relatively large proper motion and high radial velocity and is catalogued as a high proper motion star in astronomical databases.[1] Because of its unusual kinematics, it has been interpreted either as a possible runaway star ejected from the Scorpius-Centaurus association or as an evolved object belonging to the old Galactic disk.[2][3]
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| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Sextans |
| Right ascension | 09h 52m 12.16s[1] |
| Declination | +02° 27′ 14.87″[1] |
| Apparent magnitude (V) | 6.02[2] |
| Characteristics | |
| Spectral type | A0III/IV[1] |
| U−B color index | −0.09[2] |
| B−V color index | −0.04[2] |
| Astrometry | |
| Radial velocity (Rv) | 103.29±0.18[1] km/s |
| Proper motion (μ) | RA: −183.327[1] mas/yr Dec.: 94.912[1] mas/yr |
| Parallax (π) | 4.9887±0.0424 mas[1] |
| Distance | 654 ± 6 ly (200 ± 2 pc) |
| Details | |
| Mass | 3.5[3] M☉ |
| Surface gravity (log g) | 3.69[4] cgs |
| Temperature | 10,135[4] K |
| Rotational velocity (v sin i) | 22[4] km/s |
| Other designations | |
| 7 Sex, HD 85504, HIP 48414, HR 3906, SAO 117959 | |
| Database references | |
| SIMBAD | data |
Early spectrophotometric studies
An early spectrophotometric comparison between 7 Sextantis and Sirius was reported by Yvonne H. Stone and George Wallerstein in 1961.[5] Their analysis indicated that titanium and chromium appeared to have normal abundances, while magnesium, silicon and manganese might be enhanced by factors of two to three relative to Sirius.[5]
The same study suggested that strontium could be deficient by roughly a factor of two and barium by at least a factor of three.[5] The helium lines He I λ4026, λ4388 and λ4471 appeared stronger than expected, possibly indicating an excess of helium, although the authors emphasised that this conclusion was highly sensitive to the assumed temperature.[5]
A full spectroscopic analysis was subsequently published by George Wallerstein, Yvonne H. Stone, and John A. Williams in 1962.[6] Using curve of growth techniques and spectra obtained with the 100-inch coudé spectrograph at Mount Wilson Observatory, they compared 7 Sextantis directly with Sirius. Their analysis found the ratio of metals to hydrogen to be similar in both stars, while the helium-to-hydrogen ratio in 7 Sextantis was found to be at least ten times higher than in Sirius.[6] A strong spectral feature near λ4267 was tentatively identified as C II, which, if correct, would imply a carbon abundance about twenty times higher than in Sirius.[6] Most other elements, including oxygen, sodium, aluminium, silicon, calcium, titanium, vanadium, and iron, were found to be normal relative to Sirius, while strontium appeared deficient by about 0.30 dex and barium was detected only as an upper limit, suggesting a significant deficiency.[6]
The authors considered two possible interpretations of the abundance anomalies. One possibility was that 7 Sextantis is a rapid rotator seen pole-on, producing a composite spectrum that mimics abundance anomalies. The other was that the anomalies are real and that the star had evolved from a metal-poor main sequence star onto the horizontal branch, with mass loss and mixing having brought helium and carbon produced in the interior to the surface.[6]
Atmospheric parameters and chemical composition
More detailed analyses were carried out in later spectroscopic studies. Alexander Rodgers and Peter Wood derived atmospheric parameters of θeff = 0.49 ± 0.01 and log g = 3.45 ± 0.15 from spectrophotometric observations.[3] Their analysis found helium and iron abundances essentially consistent with solar values.[3] They also estimated a microturbulent velocity of about 1.8 km s−1.[3] An earlier comparison of the Hγ line profiles by Wallerstein et al. had shown these to be similar in 7 Sextantis and Sirius, indicating that the surface gravity does not differ drastically from that of a main-sequence star.[6]
A later abundance study by Saul Adelman and A. G. Davis Philip determined an effective temperature of 10,135 K and a surface gravity of log g = 3.69 for 7 Sextantis using photometric calibrations.[4] They reported a helium abundance of He/H = 0.10 ± 0.01 derived from the He I λ4472 line.[4]
Their results indicated that most elements in the atmosphere of 7 Sextantis have abundances close to solar values.[4] Excluding a somewhat uncertain sulphur measurement, the mean abundance ratio was estimated as [A/H] = 0.20 ± 0.22, suggesting the star may be slightly metal-rich.[4] Carbon and chromium were found to be enhanced by about +0.5 dex relative to the Sun.[4]
The star shows moderate rotation, with a projected rotational velocity of about 22 km s−1 reported in the abundance analysis.[4] An earlier measurement cited by Rodgers and Wood gave a value of roughly 15 km s−1.[3]
Kinematics and origin
The large proper motion and high radial velocity of 7 Sextantis have led to different interpretations of its origin.[2][3] Wallerstein et al. measured a total space velocity of about 161 km s−1 relative to the local standard of rest,[6] with 34 km s−1 directed toward the north galactic pole.[6] Martinet suggested that the star may be a runaway star ejected from the Upper Centaurus-Lupus subgroup of the Scorpius-Centaurus association.[2] By tracing its motion backward in time, he estimated that such an ejection would have occurred about 1.1-1.5 × 106 years ago.[2]
Rodgers and Wood later proposed an alternative explanation, arguing that the star's Galactic orbit was inconsistent with an evolved runaway B-star scenario.[3] Instead they interpreted the star as an old-disk horizontal-branch star with a mass of about 3.5 M☉ and an estimated age of approximately 1.7 × 108 years.[3] Their orbital calculations indicated that the star moves on an eccentric orbit characteristic of old-disk objects.[3]