HIP 114328: a new refractory-poor and Li-poor solar twin ††thanks: Based on observations obtained at the European Southern Observatory (ESO) Very Large Telescope (VLT) at Paranal Observatory, Chile (observing program 083.D-0871).
Key Words.:Sun: abundances – stars: fundamental parameters — stars: abundances – planetary systems
Context:The standard solar model fails to predict the very low lithium abundance in the Sun, which is much lower than the proto-solar nebula (as measured in meteorites). This Li problem has been debated for decades, and it has been ascribed either to planet formation or to secular stellar depletion due to additional mixing below the convection zone, either during the pre-main sequence and thus possibly linked to planet formation or additionally on secular time-scales during the main sequence. In order to test the evolution of Li, it is important to find solar twins in a range of ages, i.e., stars with about a solar mass and metallicity but in different evolutionary stages. Also, the study of stars similar to the Sun is relevant in relation to the signature of terrestrial planet formation around the Sun, and for anchoring photometric and spectroscopic stellar parameter scales.
Aims:We aim to identify and analyse solar twins using high quality spectra, in order to study Li depletion in the Sun and the possible relation between chemical abundance anomalies and planet formation.
Methods:We acquired high-resolution (R 110,000), high S/N (300) ESO/VLT UVES spectra of several solar twin candidates and the Sun (as reflected from the asteroid Juno). Among the solar twin candidates we identify HIP 114328 as a solar twin and perform a differential line-by-line abundance analysis of this star relative to the Sun.
Results:HIP 114328 has stellar parameters = 578510 K, log = 4.380.03, [Fe/H] = -0.0220.009, and a microturbulent velocity 0.050.03 km s higher than solar. The differential analysis shows that this star is chemically very similar to the Sun. The refractory elements seem even slightly more depleted than in the Sun, meaning that HIP 114328 may be as likely to form terrestrial planets as the Sun. HIP 114328 is about 2 Gyr older than the Sun, and is thus the second oldest solar twin analyzed at high precision. It has a Li abundance of A(Li) 0.46, which is about 4 times lower than in the Sun (A(Li) = 1.07 dex), but close to the oldest solar twin known, HIP 102152.
Conclusions:Based on the lower abundances of refractory elements when compared to other solar twins, HIP 114328 seems an excellent candidate to host rocky planets. The low Li abundance of this star is consistent with its old age and fits very well the emerging Li-age relation among solar twins of different ages.
Since we can observe the Sun only at its current age, we have to rely upon younger and older stars to understand how the Sun would have been or how it will be at different evolutionary stages. The ideal sample of stars to compare the Sun with are solar twins (Cayrel de Strobel, 1996). Although solar twins are usually defined based on the similarity of their spectra to the Sun (e.g., Datson et al., 2014) or stellar parameters (e.g., Ramírez et al., 2009), here we refer to solar twins as main sequence stars with about one-solar-mass and about solar composition, but spanning a range of ages. Having a mass and composition similar to the Sun ensures that solar twins will follow about the same evolutionary path as the Sun, thus allowing us to study the evolution of the Sun in time. As the stellar parameter space covered by one-solar-mass main sequence stars of roughly solar metallicity ([Fe/H] = 0.00.1 dex) is broader than the working definition of solar twins given by Ramírez et al. (2009), which is within 100K, log within 0.1 dex and [Fe/H] within 0.1 dex of the solar values, all previous solar twins are included in our definition.
Of particular importance is the study of lithium; this element has been potentially related either to planet formation (Israelian et al., 2004, 2009; Chen & Zhao, 2006; Gonzalez et al., 2010; Takeda et al., 2010; Delgado Mena et al., 2014; Gonzalez, 2014) or to stellar depletion as stars evolve (Meléndez et al., 2010; Baumann et al., 2010; Monroe et al., 2013). A sample of solar twins with a range of ages is crucial to better understand this element, which can be used as an important constraint for non-standard stellar evolution models (Charbonnel & Talon, 2005; Xiong & Deng, 2009; do Nascimento et al., 2009; Baraffe & Chabrier, 2010; Denissenkov, 2010; Li et al., 2012).
Several bright (V 10) solar twins have been identified already (Porto de Mello & da Silva, 1997; Meléndez et al., 2006; Meléndez & Ramírez, 2007; Takeda et al., 2007; Takeda & Tajitsu, 2009; Meléndez et al., 2009; Ramírez et al., 2009; Datson et al., 2012, 2014; Porto de Mello et al., 2014), so they can be subject to high S/N, high resolving power () studies, i.e. to high precision analyses using a high figure of merit (Norris et al., 2001). For example, the work by Ramírez et al. (2011) achieved a precision of about 0.01-0.02 dex in chemical abundances using , while both Meléndez et al. (2012) and Monroe et al. (2013) achieved a precision of about 0.005-0.010 dex with , and Meléndez et al. (2014) achieved a precision of about 0.005 dex with . Among those solar twins studied at high precision (F 4000), only 16 Cyg B (Ramírez et al., 2011) and HIP 102152 (Monroe et al., 2013) seem older than the Sun.
In this Letter we report the identification of another solar twin older than the Sun, HIP 114328 (HD 218544), thus bringing important insights on the evolution of Li and therefore on the mechanisms that destroy this fragile element in solar type stars. Also, we will discuss the refractory-poor abundance pattern of this star in the context of chemical anomalies and planet formation.
Based on their colors and Hipparcos parallaxes, we selected eight solar twin candidates for spectroscopic observations, HIP1536, HIP 3238, HIP 10725, HIP11514, HIP 106288, HIP 109381, HIP 114328 and HIP 117499, as well as the asteroid Juno to obtain a reference solar spectrum. The observations were taken using UVES in dichroic mode, with the 346nm setting (306-387 nm) in the blue arm and the 580-nm setting (480 - 682 nm) in the red arm.
Most spectral lines used are in the red arm, where we achieved R = 110 000 using the 0.3 arcsec slit. The typical S/N is about 285 per pixel, thus our figure of merit is F 5000. In the UV we used a slit of 0.6 arcsec, resulting in R = 65 000.
The spectral orders were extracted and wavelength calibrated using IRAF. Further data processing was performed with IDL. A comparison of the solar twin candidates to the Sun, revealed that only the spectrum of HIP 114328 matched well the solar spectrum, hence this star was selected for a further detailed analysis. Part of the reduced spectra of HIP 114328 and the Sun is shown in Fig. 1 in the region 6078-6095 Å and around the Li feature. The spectra are very similar, except for the Li feature, with HIP 114328 showing a much weaker feature than the Sun, and similar to the old solar twin HIP 102152 (Monroe et al., 2013).
3 Abundance analysis
The analysis is similar to that presented in Meléndez et al. (2012); Meléndez et al. (2014) and Monroe et al. (2013). The main difference is that now all equivalent width (EW) measurements were performed by hand, instead of first having a set of automatic measurements with ARES (Sousa et al., 2007). The number of outliers in the hand measurements is significantly smaller than those obtained in our previous works when using EWs measured automatically. Thus, only for a few lines we needed to check the manual measurements. The line list is from Meléndez et al. (2014), and is an extended version of that presented in Meléndez et al. (2012).
The same differential approach as in our previous papers was used to obtain stellar parameters and chemical abundances, i.e., we followed a strictly differential line-by-line analysis. We adopted ATLAS9 model atmospheres (Castelli & Kurucz, 2004), although the differential analysis of solar twins is essentially insensitive to the chosen grid of model atmospheres (Meléndez et al., 2012). The analysis was performed using the 2002 version of MOOG (Sneden, 1973). As shown in Meléndez et al. (2012); Meléndez et al. (2014) and Monroe et al. (2013), differential NLTE corrections in solar twins are negligible, hence they are not taken into account here.
The differential spectroscopic equilibrium of HIP 114328 relative to the Sun results in stellar parameters of = 578510 K ( = +810 K), log = 4.380.03 dex (log = 0.060.03 dex), [Fe/H] = 0.0220.009 dex, and a microturbulent velocity +0.050.03 km s higher than solar. The errors in the stellar parameters were estimated based on the observational uncertainties and take into account the degeneracy in the stellar parameters.
Once the stellar parameters were set, we computed differential abundances using the measured EWs, except for Li, which was analysed by spectrum synthesis using the line list of Meléndez et al. (2012). Hyperfine structure was taken into account for V, Mn, Co and Cu. The differential abundances are provided in Table 1, as well as the observational errors (standard errors), the errors due to uncertainties in the stellar parameters, and the total error, obtained by adding in quadrature the observational and systematic errors.
In Fig. 2 we plot the differential abundances [X/H] between HIP 114328 and the Sun (circles) as a function of equilibrium condensation temperature (T, Lodders, 2003). The abundance pattern of HIP 114328 is similar to solar and seems slightly more depleted in refractories than the Sun, as shown by the fits (solid and dashed lines). Considering the uncertainties, the refractory-to-volatile ratio in HIP 114328 is similar to solar. For comparison, the mean abundance pattern of eleven solar twins (Meléndez et al., 2009), is shown by a dot-dashed line, showing that HIP 114328 is indeed depleted in refractories.
The element-to-element scatter (0.013 dex) of the differential abundances in HIP 114328 is similar to the typical error bar of the differential abundances (0.011 dex), showing that our error bars are realistic and that we achieved a typical error of 0.01 dex.
The LTE lithium abundance in HIP 114328 is A(Li) 0.410.13, and the NLTE abundance is A(Li) 0.460.13, using the NLTE corrections of Lind et al. (2009). This low Li abundance is slightly higher than the Li abundance in the old solar twin HIP 102152 (Monroe et al., 2013), and as discussed below, reinforces the idea of secular stellar depletion.
HIP 114328 has been little studied in the literature, with only two papers reported by SIMBAD, one on stellar activity (Jenkins et al., 2011) and the other one on a list of candidates for targeted transit searches (Herrero et al., 2012). There are three earlier papers on delta Scuti stars but they are actually misidentifications, and refer to DY Peg (HD 218549) and not to HIP 114328 (HD 218544).
The abundance pattern of HIP 114328 is very similar to solar (Fig. 2) and seems even slightly more depleted in refractories than the Sun. The abundance pattern of the Sun is different from most solar twins (Meléndez et al., 2009; Ramírez et al., 2009), probably due to the formation of the terrestrial planets in the solar system (Chambers, 2010). Thus, the chemical similarity between HIP 114328 and the Sun, may suggest that HIP 114328 was as well equipped as the Sun to host rocky planets.
An alternative hypothesis to explain the Sun’s abundances anomalies is that the viewing angle of solar twins from Earth is different than the angle of the Sun when observed from Earth. However, a detailed analysis of solar spectra taken at different solar latitudes revealed no abundance differences (Kiselman et al., 2011). Another explanation put forward by Önehag et al. (2011), is that the lack of refractory elements may reflect actually that the star was born in a dense environment, as suggested by the analysis of one solar twin in the open cluster M67. Interestingly, the recent work by Adibekyan et al. (2014), shows that older stars have a lower refractory-to-volatile ratio than younger stars, thus suggesting that age may play a role in the trends with condensation temperature. However, the recent discovery of clear abundance differences between the binary components of 16 Cygni (Laws & Gonzalez, 2001; Ramírez et al., 2011; Tucci Maia et al., 2014), where the secondary hosts a giant planet but no planet has been detected around the primary despite more than two decades of radial velocity monitoring, strongly suggests that planet formation can indeed imprint chemical signatures on the composition of their host stars. Notice that although both Schuler et al. (2011) and Takeda (2005) found no abundance difference between 16 Cyg A and B, their work is based on spectra with a lower figure of merit (i.e., lower quality) than in Tucci Maia et al. (2014), who used a resolving power R = 81 000 and S/N = 700, implying = 9450 at 6000 Å. On the other hand, Schuler et al. (2011) used R = 45 000 and S/N=750, hence their = 5625. The work of Takeda (2005), made use of R = 70 000 spectra, which had a low S/N = 90 - 130 (Takeda et al., 2005), resulting in a much lower figure of merit ( = 1280).
Using our precise stellar parameters with their error bars, Yonsei-Yale isochrones (Kim et al., 2002; Demarque et al., 2004) and probability distribution functions (as described in Meléndez et al., 2012), we estimate an age and mass for HIP 114328 of 6.7 Gyr and 0.990.01 M, respectively. The old age of this solar twin is consistent with the low activity level measured by Jenkins et al. (2011), = -5.024, fitting well the activity-age relation of solar twins (Ramírez et al., 2014).
We plot the Li abundance and age of HIP 114328 in Fig. 3, together with the solar twins used by Monroe et al. (2013), which are all based on analyses with high figure of merit (F 4000) and R 60 000. In this plot we updated the age of 16 Cyg B (previously from Ramírez et al., 2011) using the most precise stellar parameters of Tucci-Maia et al. (2014), resulting in an age of 6.6 Gyr. We also plot several theoretical tracks of non-standard models of lithium depletion (Charbonnel & Talon, 2005; do Nascimento et al., 2009; Xiong & Deng, 2009; Denissenkov, 2010). HIP 114328 fits well the Li-age correlation found by Monroe et al. (2013). This connection between Li and age has been already suggested, albeit with larger uncertainties, in our earlier works (Meléndez et al., 2010; Baumann et al., 2010). This reinforces that stellar Li depletion is secular and not related to planet formation (e.g., Israelian et al., 2009); the low Li content in the Sun is perfectly normal for its age. Overall there is a good agreement with all non-standard models shown in Fig. 3.
We achieve a precision of about 0.01 dex in the differential analysis of HIP 114328 relative to the Sun. This solar twin has a chemical composition similar to solar, hence it is a good candidate to look for potential rocky planets. First identifying a Sun 2.0, or solar twin, with potential terrestrial planet formation, such as HIP 114328, is perhaps a good strategy for workers to consider as they search for an Earth 2.0, or Earth-sized planet in the habitable zone of a sun-like star. Although originally not included in our HARPS planet survey around solar twins (Ramírez et al., 2014, ESO Large Programme 188.C-0265, PI: J. Meléndez), we will start to search for planets around this star.
We determine an old age for HIP 114328 (7 Gyr), which makes it important to study the depletion of Li with age. The low Li abundance of HIP 114328 fits very well an emerging tight relation between Li and age, and shows that Li could be used as a cosmochronometer, thus helping to derive ages in main sequence stars (do Nascimento et al., 2009; Li et al., 2012). The study of more solar twins in a range of ages will help to better constrain non-standard stellar evolution models.
Acknowledgements.J.M. would like to acknowledge support from FAPESP (2012/24392-2) and CNPq (Bolsa de Produtividade). T.M. and L.S. acknowledge support from FAPESP (2010/19810-4 and 2013/25008-4). M.A and D.Y acknowledge support from the Australian Research Council (grants FL110100012 and DP120100991).
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|Element||LTE||log||[Fe/H]||paramAdding errors in stellar parameters||obsObservational errors||totalTotal error (stellar parameters and observational)|
|+10K||+0.03 dex||+0.03 km s||+0.01 dex|