# The Sun and its Stellar Twins

If you’re looking for an analog to the Sun, you have to do more than find a solitary G-class star. Three stars markedly like the Sun — 18 Scorpius, HD 98618, and HIP 100963 — still differ in having several times more lithium than our star. Figuring out whether the low amount of lithium is an unusual trait has ramifications for the search for life in the cosmos. You could theoretically push the issue by saying that the Sun’s composition is unlikely to be found elsewhere, making extraterrestrial life rare.

But that conjecture, which was a stretch to begin with, may be dampened by the recent findings about HIP 56948. 200 light years away in the constellation Draco, the star mimics the Sun’s lithium levels. And there’s an additional bonus: Bill Cochran’s team, also at McDonald, has demonstrated that HIP 56948 hosts no ‘hot Jupiters,’ giant worlds so close to their primary that they orbit in a matter of days. Thus this finding, developed using data from the 2.7-meter instrument at McDonald Observatory (University of Texas), shows us an interesting target for future terrestrial planet hunter missions, and perhaps for SETI researchers as well.

Image: HIP 56948 is more like the Sun than any known star. Located 200 light-years away in Draco, the dragon, the star is too dim to see with the unaided eye. Credit: Tim Jones/McDonald Obs./UT-Austin.

Hot Jupiters, of course, do not rule out terrestrial worlds in the same system. We know little enough about how the formation and probable migration of these planets into the inner system would affect smaller rocky worlds. But finding true Solar analogs gives us the chance to study stars like ours in a broader context, learning how accurate it is to consider Sol as the baseline for a variety of astrophysical studies. Moreover, the same team has found a second Solar analog with similar lithium levels, HIP 73815. It’s a list that’s bound to grow.

The paper is Melendez, “HIP 56948: A Solar Twin With a Low Lithium Abundance,” in press at Astrophysical Journal Letters (abstract).

Comments on this entry are closed.

• Ronald November 10, 2007, 7:00

What would be the problem with somewhat higher lithium?

Isn’t it biased to state that a star needs to have *exactly* the same composition as ours? I would rather say that a star needs to have certain characteristics, within certain margins, to be suitable (and even that may be disputable).

• andy November 10, 2007, 10:07

Ronald: depends whether what you’re interested in is habitability, or studying the star itself. If you’re interested in solar physics/chemistry, you want to have a star that’s as good a match as possible.

• ljk December 13, 2007, 10:42

The G-dwarf problem in the Galaxy

Authors: R. Caimmi

(Submitted on 12 Dec 2007 (v1), last revised 13 Dec 2007 (this version, v2))

Abstract: The empirical differential metallicity distribution (EDMD) is deduced for (i) local thick disk stars; (ii) likely metal-weak thick disk stars; (iii) chemically selected local G dwarfs, with the corrections performed in order to take into account the stellar scale height; in addition to previous results related to (iv) solar neighbourhood halo subdwarfs; and (v) K-giant bulge stars. The thick disk is conceived as made of two distinct regions: the halo-like and the bulge-like thick disk, and the related EDMD is deduced. Under the assumption that each distribution is typical for the corresponding subsystem, the EDMD of the thick disk, the thick + thin disk, and the Galaxy, is determined by weighting the mass. Models of chemical evolution are computed for each subsystem assuming the instantaneous recycling approximation. The EDMD data are reasonably fitted by simple models implying both homogeneous and inhomogeneous star formation, provided that star formation is inhibited during thick disk evolution. The initial mass function (IMF) is assumed to be a universal power law, which implies an unchanged true yield in different subsystems. The theoretical differential metallicity distribution (TDMD) is first determined for the halo-like thich disk, the bulge-like thick disk, and the thin disk separately, and then for the Galaxy by weighting the mass. An indicative comparison is performed between the EDMD deduced for the disk both in presence and in absence of [O/Fe] plateau, and its counterpart computed for (vi) nearby stars for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation. Both distributions are found to exhibit a similar trend, though systematic differences exist.

Comments: 63 pages, 13 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0712.1985v2 [astro-ph]

Submission history

From: Roberto Caimmi [view email]

[v1] Wed, 12 Dec 2007 17:32:54 GMT (229kb)

[v2] Thu, 13 Dec 2007 06:54:09 GMT (229kb)

http://arxiv.org/abs/0712.1985

• ljk February 6, 2008, 23:34

Observing solar-like oscillations

Authors: Timothy R. Bedding, Hans Kjeldsen

(Submitted on 4 Feb 2008)

Abstract: The past few years have seen great progress in observing oscillations in solar-type stars, lying on or just above the main sequence. We review the most recent results, most of which were obtained using high-precision velocity measurements. We also briefly discuss observations of more evolved stars, namely G, K and M giants and supergiants.

Comments: Proc. of a conference on “Unsolved Problems in Stellar Physics”, A Conference in Honour of Douglas Gough. AIP Conference Proceedings, Volume 948, pp. 117-124 (2007)

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.0531v1 [astro-ph]

Submission history

From: Tim Bedding [view email]

[v1] Mon, 4 Feb 2008 23:19:17 GMT (259kb)

http://arxiv.org/abs/0802.0531

• ljk March 22, 2008, 22:51

Lithium abundances in exoplanet-hosts stars

Authors: M. Castro, S. Vauclair, O. Richard, N.C. Santos

(Submitted on 20 Mar 2008)

Abstract: Exoplanet-host stars (EHS) are known to present surface chemical abundances different from those of stars without any detected planet (NEHS). EHS are, on the average, overmetallic compared to the Sun. The observations also show that, for cool stars, lithium is more depleted in EHS than in NEHS. The overmetallicity of EHS may be studied in the framework of two different scenarii.

We have computed main sequence stellar models with various masses, metallicities and accretion rates. The results show different profiles for the lithium destruction according to the scenario. We compare these results to the spectroscopic observations of lithium.

Comments: 4 pages, 5 figures, conference proceeding “XXI Century challenges for stellar evolution”, Cefalu (Sicily, Italy), August 29 – September 2, 2007

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.3014v1 [astro-ph]

Submission history

From: Matthieu Castro [view email]

[v1] Thu, 20 Mar 2008 15:53:02 GMT (829kb)

http://arxiv.org/abs/0803.3014

• ljk May 21, 2008, 11:40

A comprehensive comparison of the Sun to other stars: searching for self-selection effects

Authors: Jose A. Robles, Charles H. Lineweaver, Daniel Grether, Chris Flynn, Chas A. Egan, Michael B. Pracy, Johan Holmberg, Esko Gardner

(Submitted on 19 May 2008)

Abstract: If the origin of life and the evolution of observers on a planet is favoured by atypical properties of a planet’s host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify eleven maximally-independent properties that have plausible correlations with habitability, and that have been observed by, or can be derived from, sufficiently large, currently available and representative stellar surveys.

By comparing solar values for the eleven properties, to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95 -/+ 2% of nearby stars and its orbit around the Galaxy is less eccentric than 93 +/- 1% of FGK stars within 40 parsecs. Despite these apparently atypical properties, a chi^2 -analysis of the Sun’s values for eleven properties, taken together, yields a solar chi^2 = 8.39 +/- 0.96.

If a star is chosen at random, the probability that it will have a lower value (be more typical) than the Sun, with respect to the eleven properties analysed here, is only 29 +/- 11%. These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.

Comments: Accepted for publication in ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0805.2962v1 [astro-ph]

Submission history

From: Jose Robles A [view email]

[v1] Mon, 19 May 2008 22:35:56 GMT (391kb)

http://arxiv.org/abs/0805.2962

• ljk July 2, 2008, 13:15

Solar twins in M67

Authors: L. Pasquini, K. Biazzo, P. Bonifacio, S. Randich, L. Bedin

(Submitted on 1 Jul 2008)

Abstract: The discovery of true solar analogues is fundamental for a better understanding of the Sun and of the solar system. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very similar to those of the Sun.

We analyze FLAMES spectra of a large number of M67 main sequence stars to identify solar analogues in this cluster.We first determine cluster members which are likely not binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on the determination of stellar effective temperature, using analyses of line-depth ratios and H$\alpha$ wings, making a direct comparison with the solar spectrum obtained with the same instrument. We also compute the lithium abundance for all the stars.

Ten stars have both the temperature derived by line-depth ratios and H$\alpha$ wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening $E(B-V)=0.041$, the solar colour $(B-V)_\odot=0.649\pm0.016$ and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars have also a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution.

We find several candidates for the best solar analogues ever. These stars are amenable to further spectroscopic investigations and planet search. The solar colours are determined with rather high accuracy with an independent method, as well as the cluster distance modulus.

Comments: 13pages and 8 figures, accepted for publication in A&A

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0807.0092v1 [astro-ph]

Submission history

From: Luca Pasquini [view email]

[v1] Tue, 1 Jul 2008 09:39:01 GMT (92kb)

http://arxiv.org/abs/0807.0092

• Masit Amidu October 5, 2009, 8:52

A recently paper put serious constraints on the mass and age of
solar twins. It is a very nice study and could be useful for the fiture
sample of solar twins.

Age and mass of solar twins constrained by lithium abundance

J.D. do Nascimento Jr, M. Castro, J. Melendez, M. Bazot, S. Theado, G. F. Porto de Mello, J.R. De Medeiros

(Submitted on 23 Apr 2009 (v1), last revised 12 May 2009 (this version, v2))
We analyze the non-standard mixing history of the solar twins HIP 55459, HIP 79672, HIP 56948, HIP 73815, and HIP 100963, to determine as precisely as possible their mass and age. We computed a grid of evolutionary models with non-standard mixing at several metallicities with the Toulouse-Geneva code for a range of stellar masses assuming an error bar of +-50K in Teff. We choose the evolutionary model that reproduces accurately the observed low lithium abundances observed in the solar twins. Our best-fit model for each solar twin provides a mass and age solution constrained by their Li content and Teff determination. HIP 56948 is the most likely solar-twin candidate at the present time and our analysis infers a mass of 0.994 +- 0.004 Msun and an age of 4.71 +-1.39 Gyr. Non-standard mixing is required to explain the low Li abundances observed in solar twins. Li depletion due to additional mixing in solar twins is strongly mass dependent. An accurate lithium abundance measurement and non-standard models provide more precise information about the age and mass more robustly than determined by classical methods alone.

http://arxiv.org/abs/0904.3580