Spectroscopy of the W Virginis star V1 (K 307 ) in the globular cluster M12

High-resolution CCD spectra have been obtained for the first time for the W Virginis star V1 (K 307) in the globular cluster M12 and its closest neighbor K 307b (m _pg=14^m; the angular distance from the W Vir star is δ<1 arcsec). We determined the fundamental parameters (T _eff=5600 K, logg=1.3, and T _eff=4200 K, logg=1.0 for the W Vir star and the neighboring star, respectively) and their detailed chemical composition. The derived metallicities of the two stars ([Fe/H]=?1.27 and ?1.22 relative to the solar value) are in good agreement with the metallicities of other cluster members. Changed CNO abundances were found in the atmosphere of the W Vir star: a small carbon overabundance, [C/Fe]=+0.30 dex, and a large nitrogen overabundance, [N/Fe]=+1.15 dex, with oxygen being underabundant, [O/Fe]≈?0.2 dex. The C/O ratio is ≥1. Na and the α-process elements Mg, Al, Si, Ca, and Ti are variously enhanced with respect to iron. We found an enhanced abundance of s-process metals relative to iron: [X/Fe]=+0.34 for Y, Zr, and Ba. The overabundance of the heavier metals La, Ce, and Nd with respect to iron is larger: [X/Fe]=+0.49. The largest overabundance was found for the r-process element europium, [Eu/Fe]=+0.82. The spectrum of the W Vir star exhibits the Hα and Hβ absorption-emission profiles and the He I λ5876 Å emission line, which are traditionally interpreted as a result of shock passage in the atmosphere. However, the radial velocities determined from absorption and emission features are in conflict with the formation pattern of a strong shock. The high luminosity log L/L _⊙ = 2.98, the chemica peculiarities, and the spectral peculiarity are consistent with the post-AGB evolution in the instability strip. The pattern of relative elemental abundances [X/Fe]in the atmosphere of the neighboring star K 307b is solar. Statistically significant differences were found only for sodium and α-process elements: the mean overabundance of light metals is [X/Fe]=+0.35.     

FG Sge at the stage of dust shell ejection

New photometric observations of the variable star FG Sge, a rapidly evolving planetary nebula nucleus, were performed in 2003–2008. On 230 nights, we obtained 86 UBV and 155 BV RI (or R _c , I _c ) magnitude estimates. The maximum amplitude of the V-band light variations was >8 ^m . Six deep minima and four high maxima were observed. Analysis of the light curve has shown that the pulsation period of the star remained constant since 1991 and was P = 115 days. We have studied the wavelength dependence of the extinction at various phases of the light curve. The blueing of the B-V color at deep minima is interpreted as the result of light scattering in the circumstellar dust shell of the star formed by preceding dust ejections since 1992. Our spectroscopic observations performed on nine nights in 2003–2007 with the 125-cm telescope at the Crimean Station of the Sternberg Institute have confirmed the previously detected intensity variations of the Swan bands and the sodium doublet with brightness. It is noted that the Swan bands originate in the upper atmosphere, the star’s extended envelope, while the sodium doublet originates mainly in the circumstellar shell of FG Sge. We suggest that the star is currently located in the temperature-luminosity diagram at the turning point of the horizontal track of cooling in the direction of hot stars—evolution caused by the last helium shell flash at the planetary nebula stage.     

New spectroscopic observations of the post-AGB star V354 Lac = IRAS 22272+5435

The strongest absorption features with the lower-level excitation potentials χ _low < 1 eV are found to be split in the high-resolution optical spectra of the post-AGB star V354 Lac taken in 2007–2008 with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. Main parameters, T _eff =5650 K, log g=0.2, ξ _t =5.0 km/s, and the abundances of 22 chemical elements in the star’s atmosphere are found. The overabundance of the s-process chemical elements (Ba, La, Ce, Nd) in the star’s atmosphere is partly due to the splitting of strong lines of the ions of thesemetals. The peculiarities of the spectrum in the wavelength interval containing the LiI λ 6707 Å line can be naturally explained only by taking the overabundances of the CeII and SmII heavy-metal ions into account. The best agreement with the synthetic spectrum is achieved assuming ?(LiI)=2.0, ?(CeII)=3.2, and ?(SmII)=2.7. The velocity field both in the atmosphere and in the circumstellar envelope of V354 Lac remained stationary throughout the last 15 years of our observations.     

High Resolution Optical Spectroscopy of Hot Post-AGB Star Candidates LS IV-04 1 and LB3116

We present LTE analysis of high resolution optical spectra for B-type hot PAGB stars LS IV-04 1 and LB3116 (LSE 237). The spectra of these high Galactic latitude stars were obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph. The standard 1D LTE analysis with line-blanketed LTE model atmospheres and spectral synthesis provided fundamental atmospheric parameters of T_eff= 15 000±1000 K, log g= 2.5±0.2, ξ = 5.0±1.0 km s^?1, [M/H] = ?1.81 dex, and v sin i= 5 km s^?1 for LSIV-04 1 and T_eff= 16 000±1000 K, log g= 2.5±0.1, v sin i= 25 km s^?1, and [Fe/H] = ?0.93 dex for LB 3116. Chemical abundances of ten different elements were obtained. For LS IV-04 1, its derived model temperature contradicts with previous analysis results. The upper limits for its nitrogen and oxygen abundances were reported for the first time. The magnesium, silicon and calcium were overabundant (i.e. [Mg/Fe] = 0.8 dex, [Si/Fe] = 0.5 dex, [Ca/Fe] = 0.9 dex). With its metal-poor photosphere and V_LSR ≈ 96 km s^?1, LSIV-04 1 is likely a population II star and most probably a PAGB star. LTE abundances of LB 3116 were reported for the first time. The spectrum of this helium rich star shows 0.9 dex enhancement in the nitrogen. The photosphere of the star is slightly deficient in Mg, Si, and S. (i.e. [Mg/Fe] = ?0.2 dex, [Si/Fe] = ?0.4 dex, [S/Fe] = ?0.2 dex). The Al is slightly enhanced. The phosphorus is overabundant, i.e. [P/Fe] ≈ 1.7 ± 0.47 dex, hence LB3116 may be the first example of a PAGB star which is rich in phosphorus. With its high radial velocity (i.e.V_LSR = 73 km s^?1), and the deficiencies observed in C, Mg, Si, and S indicate that LB 3116 is likely a hot PAGB star at high galactic latitude.     

A model of low-mass neutron stars with a quark core

We consider an equation of state that leads to a first-order phase transition from the nucleon state to the quark state with a transition parameter λ>3/2 (λ=ρ_Q/(ρ_N+P₀/c²)) in superdense nuclear matter. Our calculations of integrated parameters for superdense stars using this equation of state show that on the stable branch of the dependence of stellar mass on central pressure dM/dP_c>0) in the range of low masses, a new local maximum with M_max=0.082_⊙ and R=1251 km appears after the formation of a toothlike kink (M=0.08M_⊙, R=205 km) attributable to quark production. For such a star, the mass and radius of the quark core are M_core=0.005M_⊙ and R_core=1.73 km, respectively. In the model under consideration, mass accretion can result in two successive transitions to a quark-core neutron star with energy release similar to a supernova explosion: initially, a low-mass star with a quark core is formed; the subsequent accretion leads to configurations with a radius of ～1000 km; and, finally, the second catastrophic restructuring gives rise to a star with a radius of ～100 km.     

Infrared photometry for two RV Tau stars and V1027 Cyg

We present and discuss the results of our long-term JHKLM photometry for two RV Tau stars (R Sge and RV Tau) and the yellow supergiant V1027 Cyg, a candidate for protoplanetary nebulae. The amplitude of the infrared brightness variations in R Sge and RV Tau over fourteen years of observations was 0 _. ^m 9?1 ^m ; the infrared brightness variations in V1027 Cyg over eighteen years did not exceed 0 _. ^m 25. The infrared brightness and color of R Sge fluctuated about their gradually changing mean values; the infrared brightness variations agree with a period of 70.77 days. The periodic J brightness and J-H color variations in R Sge can be explained by temperature pulsations with ΔT ≤ 200 K and radial pulsations with [ΔR/R] ≤ 0.2. From 1995 to 2008, the mean J brightness of RV Tau increased, while its mean J-H color index decreased; the variations in the mean J brightness can be associated mainly with stellar temperature variations; a periodic component with P = 78.73 days is observed in the infrared brightness and color fluctuations. The variations in the mean J brightness and J-H color index of the supergiant V1027 Cyg over eighteen years of observations did not exceed a few hundredths of a magnitude; both temperature and radial pulsations may be present in the observed J brightness variations. The most probable period of the infrared brightness fluctuations in V1027 Cyg is 237 ± 2 days. The dust shell of R Sge may consist of two layers with grain temperatures of ～1000 and ～700 K; the optical depth at 1.25 µm is ～0.02 and ～0.24, respectively. The grain temperatures in the circumstellar dust shells of the supergiants RV Tau and V1027 Cyg are ～600 K (RV Tau) and ～700 K (V1027 Cyg). Their optical depths at 1.25 µm are ～0.24 (RV Tau) and ～0.008 (V1027 Cyg).     

GJ 900: A new hierarchical system with low-mass components

Speckle interferometric observations made with the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in 2000 revealed the triple nature of the nearby (π _Hip = 51.80 ± 1.74 mas) low-mass young (≈ 200 Myr) star GJ 900. The configuration of the triple system allowed it to be dynamically unstable. Differential photometry performed from 2000 through 2004 yielded I- and K-band absolute magnitudes and spectral types for the components to be I _A =6.66±0.08, I _B =9.15±0.11, I _C =10.08±0.26, K _A =4.84±0.08, K _B =6.76±0.20, K _C =7.39±0.31, Sp _A ≈K5?K7, Sp _B ≈M3?M4, Sp _C ≈M5?M6. The “mass-luminosity” relation is used to estimate the individual masses of the components: M _A ≈0.64M _⊙, M _B ≈0.21M _⊙, M _C ≈0.13M _⊙. From the observations of the components’ relative motion in the period 2000–2006, we conclude that GJ 900 is a hierarchical triple star with the possible orbital periods P_A-BC≈80 yrs and P_BC≈20 yrs. An analysis of the 2MASS images of the region around GJ 900 leads us to suggest that the system can include other very-low-mass components.     

A preliminary radial-velocity curve for the star <Emphasis Type="Italic">θ</Emphasis><Superscript>1</Superscript> Ori D

We measured the radial velocity of the star θ¹ Ori D from IUE spectra and used published observations. Based on these data, we determined the period of its radial-velocity variations, P=20.2675±0.0010 days, constructed the phase radial-velocity curve, and solved it by least squares. The spectroscopic orbital elements were found to be the following: the epoch of periastron passage E_p=JD 2430826.6±0.1, the system's center-of-mass velocity /Gg=32.4±1.0 km s^?1, K=14.3±1.5 km s^?1, Ω=3.3±0.1 rad, e=0.68±0.09, a₁ sin i = 3 × 10¹⁰ km, and f₁ = 0.0025M_⊙. Twice the period, P=40.528±0.002 days, is also consistent with the observations.     

Stellar population of the blue compact dwarf galaxy SBS 1415+437

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated using the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 ^m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch αTRGB) was used to calculate the distance modulus, which turned out to be m ? M = 30.65 ± 0.08 ^m . The corresponding distance to the galaxy is D = 13.5 ± 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old population occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distribution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 × 10⁷ yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V ? I) _TRGB ≈ 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10¹⁰ yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is [Fe/H] = ?2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to [Fe/H] = ?1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 × 10⁹ yr.     

A study of the B[e] star AS 160

We present the results of our study of the poorly known B[e] star AS 160=IRAS 07370-2438. The high-resolution spectrum obtained with the 6-m BTA telescope exhibits strong emission in the Hα line with a two-component profile, indicating that the gaseous envelope of the star is nonspherical. Previously nonanalyzed photometric data suggest the presence of a compact dust envelope. The fundamental parameters of the star (log L/L_⊙ = 4.4 ± 0.2, v sin i = 200 km s^?1 and its distance (3.5±0.5 kpc) have been determined for the first time and are in agreement with published estimates of the MK spectral type of the object (B1.5 V:). Analysis of the object's properties leads us to suggest that this is a binary system that belongs to our recently identified type of Be stars with warm dust.     

Stars with Discrepant <Emphasis Type="Italic">v</Emphasis> sin <Emphasis Type="Italic">i</Emphasis> as Derived from the Ca II 3933 and Mg II 4481 Å Lines. VII. HD9531 (SB), HD31592 (SB2), HD129174 (SB?)

In this paper of the series we analyze three stars listed among stars with discrepant v sin i: HD9531 and HD31592, which also show radial velocity variations inherent to spectroscopic binaries, and HD129174 which is an Mn-type star with a possible magnetic field. In HD9531 we confirm the radial velocity derived fromthe hydrogen lines as well as fromthe Ca II line at 3933 Å as variable. The profile of the calcium line also appears variable, and with the estimated magnetic induction B_e = ?630 ± 1340 G, this suggests that the abundance of calcium possibly varies over the surface of the star. We identified the lines of the secondary component in the spectrum of HD31592 revealing thus it is an SB2 binary with B9.5V and A0V components. While the primary star rotates with v sin i = 50 km s^?1, the secondary star is faster with v sin i = 170 km s^?1. We find that only 60% of the Mn lines identified in the spectrum of HD129174 can be fitted with a unique abundance value, whereas the remaining lines are stronger or fainter. We also identified two Xe II lines at 5339.33 Å and 5419.15 Å and estimated their log g f.     

Magnetic stars with wide depressions in the continuum. 1. The Ap star with strong silicon lines HD5601

Based on observations with the 6-m SAO RAS telescope, we have found that chemically peculiar star with a large depression of the continuum at λ5200 Å and strengthened silicon lines in the spectrum has a strong magnetic field. The longitudinal field component B_e has a negative polarity and varies from ?300 G to ?2000 G with a period of 1.756 days. Photometric variations of brightness take place with the same period. We determined the variability of the radial velocity at times of about tens of years pointing to a possible binarity of the object. We have built a magnetic model of this star, determined the inclination angles of the rotation axis to the line of sight i = 20° and of the dipole axis to the rotation axis β = 116°, and the field strength at the pole is B_p = 10 kG. We carried out a chemical composition analysis and found a lack of helium for almost an order of magnitude, some overabundance of silicon and metal elements for more than an order of magnitude, particularly, cobalt for three orders of magnitude.     

Analysis of HST ultraviolet spectra for T Tauri stars: DR Tau

We analyze the spectra of DR Tau in the wavelength range 1200 to 3100 Å obtained with the GHRS and STIS spectrographs from the Hubble Space Telescope. The profiles for the C IV 1550 and He II 1640 emission lines and for the absorption features of some lines indicate that matter falls to the star at a velocity ～300 km s^?1. At the same time, absorption features were detected in the blue wings of the N I, Mg I, Fe II, Mg II, C II, and Si II lines, suggesting mass outflow at a velocity up to 400 km s^?1. The C II, Si II, and Al II intercombination lines exhibit symmetric profiles whose peaks have the same radial velocity as the star. This is also true for the emission features of the Fe II and H₂ lines. We believe that stellar activity is attributable to disk accretion of circumstellar matter, with matter reaching the star mainly through the disk and the boundary layer. At the time of observations, the accretion luminosity was L_ac ? 2L_⊙ at an accretion rate ?10^?7M_⊙ yr^?1. Concurrently, a small (<10%) fraction of matter falls to the star along magnetospheric magnetic field lines from a height ～R_*. Within a region of size ?3.5R_*, the disk atmosphere has a thickness ～0.1R_* and a temperature ?1.5 × 10⁴ K. We assume that disk rotation in this region significantly differs from Keplerian rotation. The molecular hydrogen lines are formed in the disk at a distance <1.4 AU from the star. Accretion is accompanied by mass outflow from the accretion-disk surface. In a region of size <10R_*, the wind gas has a temperature ～7000 K, but at the same time, almost all iron is singly ionized by H I L _α photons from inner disk regions. Where the warm-wind velocity reaches ?400 km s^?1, the gas moves at an angle of no less than 30° to the disk plane. We found no evidence of regions with a temperature above 10⁴ K in the wind and leave open the question of whether there is outflow in the H₂ line formation region. According to our estimate, the star has the following set of parameters: M_* ? 0.9M_⊙, R_* ? 1.8R_⊙, L_* ? 0.9L_⊙, and \(A_V \simeq 0\mathop .\limits^m 9\). The inclination i of the disk axis to the line of sight cannot be very small; however, i≤60°.     

Stars with discrepant <Emphasis Type="Italic">υ</Emphasis> sin <Emphasis Type="Italic">i</Emphasis> as derived from Ca II 3933 and Mg II 4481 Å lines. II. Multiple star HD 90569

Axial rotation of a star plays an important role in its evolution, physical condition in its atmosphere and appearance of its spectrum. Methods of determinations of υ sin i are based on comparison of the observed profiles of spectral lines with theoretical ones. Their accuracy depends on the kind and quality of spectrograms as well as on the algorithms used. A frequently used method is the simple comparing of one line, e.g. the Ca II at 3933 Å or Mg ii at 4481 Å. This however, may result in a false value of υ sin i if low dispersion spectra are used. In this work we studied contemporary CCD as well as older photographic spectra of the multiple star HD90569. We determined the projected rotational velocity value to be υ sin i = 11 km/s. Besides formerly reported enhancing of lines of Cr, Fe, Mn and Sr, we found also large overabundances of rare earths, gallium and platinum. Helium, carbon, nitrogen, oxygen, aluminium, calcium, scandium and nickel are in deficit. The spectrum of the occultation double was not identified to be of the SB2-type, however, there are some observable evidences that the pair creates a binary with a long orbital period. Despite this there are also observations that leave such interpretation uncertain.     

On a family of well behaved perfect fluid balls as astrophysical objects in general relativity

A family of well behaved perfect fluid balls has been derived starting with the metric potential g ₄₄=B(1+Cr ²) ⁿ for all positive integral values of n. For n≥4, the members of this family are seen to satisfy the various physical conditions e.g. c ² ρ≥p≥0,dp/dr<0,dρ/dr<0, along with the velocity of sound \((\sqrt{dp/c^{2}d\rho} )< 1\) and the adiabatic index ((p+c ² ρ)/p)(dp/(c ² dρ))>1. Also the pressure, energy density, velocity of sound and ratio of pressure and energy density are of monotonically decreasing towards the pressure free interface (r=a). The fluid balls join smoothly with the Schwarzschild exterior model at r=a. The well behaved perfect fluid balls so obtained are utilised to construct the superdense star models with their surface density 2×10¹⁴  gm/cm³. We have found that the maximum mass of the fluid balls corresponding to various values of n are decreasing with the increasing values of n. Over all maximum mass for the whole family turns out to be 4.1848M _Θ and the corresponding radius as 19.4144 km while the red shift at the centre and red shift at surface as Z ₀=1.6459 and Z _a =0.6538 respectively this all happens for n=4. It is interesting to note that for higher values of n viz n≥170, the physical data start merging with that of Kuchowicz superdense star models and hence the family of fluid models tends to the Kuchowicz fluid models as n→∞. Consequently the maximum mass of the family of solution can not be less than 1.6096 M _Θ which is the maximum mass occupied by the Kuchowicz superdense ball. Hence each member of the family for n≥4 provides the astrophysical objects like White dwarfs, Quark star, typical neutron star.     

Magnetic field and chemical composition of the peculiar star HD 10221

We analyzed the chemical composition of the chemically peculiar (CP) star HD 0221=43 Cas using spectra taken with the NES spectrograph of the 6-m telescope with a spectral resolution of 45 000. The Hβ line profile corresponds most closely to T_eff = 11 900 K and log g = 3.9. The rotational velocity is v_e sin i = 27 ± 2 km s^?1, and the microturbulence is ξ_t = 1 km s^?1. The results of our abundance determination by the method of synthetic spectra show that the star has chemical anomalies typical of SrCrEu stars, although its effective magnetic field is weak, B_e < 100 G. For silicon, we obtained an abundance distribution in atmospheric depth with a sharp jump of 1.5 dex at an optical depth of log τ₅₀₀₀ = ?0.3 and with silicon concentration in deep atmospheric layers. Similar distributions were found in the atmospheres of cooler stars with strong and weak magnetic fields. A comparison of the chemical peculiarities in HD 10221 with known CP stars with magnetic fields of various strengths leads us to conclude that a low rotational velocity rather than amagnetic field is the determining factor in the formation mechanism of chemical anomalies in the atmospheres of CP stars.     

Variability and rapid evolution of the protoplanetary object IRAS 18062+2410=V886 her

We present our long-term photometric and spectroscopic observations of a high-latitude B supergiant with an infrared excess—the protoplanetary nebula IRAS 18062+2410. OurU BV observations in 2000–2006 have confirmed the rapid irregular photometric variability of the star with a maximum amplitude as high as 0 _. ^m 4 in V that we found previously. The B—V and U—B color indices vary with amplitudes as high as 0 _. ^m 10 and 0 _. ^m 25, respectively, and show no clear correlation with the brightness. Our V-band CCD observations on 11 nights in 2006 have revealed brightness trends during the night. The variability of IRAS18062+2410 is similar in pattern to the light variations in other hot post-AGB objects and some of the nuclei of young planetary nebulae. We assume that pulsations and a variable stellar wind can be responsible for the variability of these stars. In addition to the rapid variability, our 12-year-long observations have revealed a systematic decline in the mean brightness of IRAS 18062+2410. This may be related to a rise in the temperature of the star at constant luminosity as a result of its evolution. Low-resolution spectroscopic observations have shown a systematic increase in the equivalent widths of the Hα, Hβ, [NII]λ6584 Å, OI λ8446 Å, and [OII] λ7320–7330 Å emission lines. The changes in the star’s emission line spectrum are probably caused by an increase in the degree of ionization of the gas shell due to a rise in the temperature of the ionizing star. Our photometric and spectroscopic observations of IRAS 18062+2410 confirm the previously made assumptions that the star evolves very rapidly to the region of planetary nebulae.     

IR photometry results and dust envelope model for symbiotic Mira star candidate V 335 Vul

We present the results of JHKLM-photometry for the symbiotic Mira star candidate V 335 Vul. Based on the average flux data, supplemented by IRAS, MSX, AKARI, and WISE mid-IR observations, we calculated a model of a spherically symmetric dust envelope of the star, made up of amorphous carbon and silicon carbide particles. The optical depth of the envelope in the visible range with a dust temperature at the inner boundary of T₁ = 1300 K is τ _V = 0.58. For an envelope expansion velocity of 26.5 km s^?1, the estimated mass loss rate is equal to 5.7 × 10^?7M_⊙ yr^?1.     

Irregular dwarf galaxy KK 230: A relatively young galaxy at the periphery of the local group?

We present some results of the photometric analysis of the stellar population of the irregular dwarf galaxy KK 230 on the basis of the archive database of the Hubble space telescope. The color index-magnitude diagram for KK 230 gets to magnitude 27 ^m in the V and I bands, and it comprises stellar populations of various ages. The age of the youngest main-sequence stars is 3.2 × 10⁷ yr. These stars are distributed along the north-south direction in the picture plane, and this fact can be linked to the observed kinematics of the neutral gas in the galaxy. Older blue and red supergiants are no less than 1.6 × 10⁸ years old, and such an age implies that the star formation was episodic over the last several hundreds of millions of years. As judged from the position of the tip of the red giant branch, the distance modulus for KK 230 is m ? M = 26.5 ^m . The corresponding distance is D = 2 Mpc. Based on the average absolute magnitude M _I,RC and color index (V ? I)_I,RC of the red clump, we conclude that the majority of KK 230 stars have an age of no more than (2–3) × 10⁹ yr, their metallicity being Z ≈ 0.0004.