SDSS J001820.5-093939.2

SDSS J001820.5–093939.2

First 2nd generation star
Observation data
Epoch J2000      Equinox J2000
Constellation Cetus
Right ascension 00h 18m 20.5s
Declination −09° 39′ 39.2″
Apparent magnitude (V) 15.8
Characteristics
Evolutionary stage main sequence star
Spectral type F9
Variable type None
Astrometry
Distance1,000 ly
(300 pc)
Absolute magnitude (MV)8.0
Orbit
Details
Mass0.47 M☉
Temperature4600 K
Metallicity [Fe/H]–2.5 dex
Age~13+ Gyr
Other designations
SDSS J001820.5-093939.2, SDSS J0018-0939, J0018-0939
Database references

SDSS J001820.5–093939.2 or SDSS J0018-0939 for short is a star system approximately 1000 light-years away near the constellation Cetus.

SDSS J0018-0939 is a cool main-sequence star. It is the first star found to be a second generation star.[1]

Background

Spectrum of SDSS J0018-0939 provided by the Sloan Digital Sky Survey.

Theory and simulations have predicted the formation of massive stars, within a few hundred million years after the Big Bang, from gas clouds containing only hydrogen and helium. As supernova explosions ejected elements formed by the first massive stars and dispersed them into the gas that formed the next generations of stars, the composition of a star is an indirect indication of the previous generation. The mass distribution of the first generation stars is key to understanding the formation of the Universe's structure and chemical enrichment; large stellar systems like galaxies. Yet, no evidence of supernovae from such very massive stars has been found in the chemical compositions of Milky Way stars.

Stars with masses less than the Sun's have very long lifetimes, long enough to be found. The distinctive chemical abundance patterns of these stars can be used to estimate the masses of the first stars. Over the past thirty years astronomers have conducted large-scale investigations to find low-mass, metal-poor stars formed in the early Universe.[2] The recent Sloan Digital Sky Survey (SDSS) and Sloan Extension for Galactic Understanding and Exploration (SEGUE) projects were the latest to provide evidence for the age, composition and phase space distribution of stars within the various Galactic components, providing crucial clues for understanding the structure, formation and evolution of our Galaxy.

SDSS J0018-0939 was identified as a star likely to have a very low abundance of metals. Many other metal-poor stars have been identified as members of dwarf galaxies around the Milky Way. Most of them are not as metal-poor and do not share other properties, suggesting that the origin these instances is different.

SDSS J0018-0939 has no signature of extra mixing or mass transfer across a binary system, which could have changed its composition. As an unevolved star internal mixing had not happened yet. Its lighterelement abundance ratios, including C and Mg are exceptionally low. Its abundance ratios between adjacent odd- and evenelement pairs are very low, which is clear in comparison with the values for G39-36, used for comparison. The upper limits on the abundance of the heavy neutron-capture elements Sr and Ba are anomalously low compared to other stars with similar metallicity. This feature is sometimes found in more metal-deficient stars ([Fe/H] < –3). Although, the Fe abundance is not as low as extremely metal-poor stars, the lowabundances of C, Mg, and the heavy neutron-capture elements (Sr and Ba) suggest that it is a very chemically primitive object.[1]

A team of astronomers from the National Astronomical Observatory of Japan (NAOJ), the Konan University and the University of Hyogo in Japan, the University of Notre Dame, and New Mexico State University used the 8.2 m Subaru Telescope's High Dispersion Spectrograph (HDS) to study SDSS J0018-0939 in more detail.[2]

Nucleosynthesis models for supernova explosions of massive stars, which confirmed previously found early-generation stars did not readily explain the chemical abundance ratios observed in SDSS J0018-0939. However, explosion models of very-massive stars with more than 100 solar masses have shown synthesis of large amounts of iron but little of lighter elements, e.g. carbon. This means that SDSS J0018-0939 most likely preserved the elemental abundance ratios produced by a first-generation very-massive star.[2]

First generation stars are expected to self-regulate their growth by radiative feedback in the formation process, and to achieve masses typically tens of times that of the Sun. A fraction of stars might have become very-massive objects, with Mms > 300 M☉.[1]

Such a star enters the pair-instability region during its evolution but continues to collapse and finally enters an instability region with Fe photodisintegration. Such objects are called core-collapse very massive stars. Although it is not clear whether or not such a very massive star can explode, the yield of an explosion with energy of about 6 ×1053 ergs can simultaneously explain both the low Si abundance (compared with Mg) and the low C and Mg abundances.[1]

A star with 140 M☉ ≲ Mms ≲ 300 M☉ explodes because of the energy consumption arising from an electron-positron pair-production instability during the static O-burning stage, and is referred to as a pair-instability supernova (PISN). Theoretical estimates of early chemical enrichment predict that the metallicity produced by the PISN explosions of a first generation of very massive stars matches the Fe abundance of SDSS J0018-0939. They also predict that stars formed from gas enriched by PISN are quite rare; only one star among 500 stars. Although about 500 stars in the metallicity range –3< [Fe/H]<–2 have been observed to date with high-resolution spectroscopy, SDSS J0018-0939 is unique in its observed abundance pattern. No other similar object has been found yet.[1]

If SDSS J0018-0939 indeed records the yields of a PISN or the explosion of a very massive star, the number fraction of very massive stars among primordial stellar populations could be several percent, which is comparable to that predicted by recent theoretical studies on the formation of first generation stars. And this could be related to that of its natal dark-matter halo.[1]

The strong UV radiation, energetic explosions, and production of heavy elements from very-massive stars influence subsequent star as well as galaxy formation. If stars with masses up to 1000 solar masses existed, their remnants are probably black holes with several hundred solar masses, which may have formed the "seeds" of super-massive black holes, such as found in the Galactic Center.[2]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Wako Aoki; Nozomu Tominaga; Timothy C. Beers; Satoshi Honda et al. (August 22, 2014). "A chemical signature of first-generation very massive stars". Science 345 (6199): 912–915. doi:10.1126/science.1252633.
  2. ↑ 2.0 2.1 2.2 2.3 Press Release (August 21, 2014), A Chemical Signature of First-Generation Very-Massive Stars, Subaru Telescope

External links

Coordinates: 00h 18m 20.5s, −09° 39′ 39.2″