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Appendix A: STELLA/WiFSIP observing log, transformation functions, and combined CCD images

Fig. A.1 STELLA/WiFSIP fields for IC 4756. Six fields were observed, indicated by the dashed boxes. Field f2 in the top left corner had been offset due to an intervening bright star. Each field is 22′×22′ and the total coverage is 0.67 sq. deg or 38% of the cluster. The approximate size of the cluster (90′) is indicated by the large circle on the left. The cluster center is at 18h38m31.2s, +05°29′24′′ (2000.0) and is highlighted by the rotated square box. The stellar ID numbers are the running numbers from Table 1 and identify the stars observed with CoRoT. The image is a uby composite.

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Table A.1 is the observing log of the Strömgren photometry with the Wide Field STELLA Imaging Photometer (WiFSIP). Only observing blocks are identified, not the individual CCD frames. One observing block consists of five consecutive integrations in u, v, b, y, Hαn, Hαw, Hβn, and Hβw. The table lists the mean JD given for the mid exposure, the field identification from Fig. A.1, the total number of CCD frames, total exposure time, the FWHM of the average stellar image, and the average airmass of the block.

The transformation equations are based on the calibration of Crawford & Barnes (1970) and we followed the recipe from Balog et al. (2001). Its four zero-point constants refer to the slopes for (b−y), m₁, c₁ and β, respectively, while the three color-term coefficients are for V, m₁ and c₁. The WiFSIP field of view contained eight standard stars from the compilation of Hauck & Mermilliod (1998) and six of them could be used to determine the zero points. However, all of these stars are brighter than 9th magnitude in V and are thus close to the saturation limit of our CCD. Therefore, the standard deviations between the literature values and the WiFSIP instrumental values are comparablylarge, ± 0.012 for (b−y), ±0.07 for c₁, ±0.04 for m₁ and ±0.19 for β, and are not representative for the photometric precision. The (b−y) color term in the equations for m₁ and c₁ was too uncertain and was set to zero. A.1The final WiFSIP Strömgren and Hα indices for the CoRoT stars are listed in Table A.2.

The epoch stacks were astrometrically re-calibrated in the tangent plane projection with distortion polynomials using sources detected in Johnson H or K_s from the 2MASS catalog (Cutri et al. 2003), and those with Johnson . An IRAF script iteratively invokes CCFIND, which correlates the catalog sources with those in the image, and CCMAP, which computes the astrometric solution. The Legendre fit to the projection transformation was cubic, with a threshold of 2.4 σ to reject outlier astrometric points. The resulting projection transformation had a standard deviation typically of ≲0.10′′ in both right ascension and declination.

The combined images for each bandpass represent very deep integrations with an equivalent exposure time of, e.g., 3900 s in y and up to 10 740 s in narrow-band Hα. Thereby, the IRAF FINDSTAR routine identified the following numbers of stars per CCD field in the y band: f2 29 102; f3 16 152; f5 16 302; f6 19 607; f8 22 870; and in f9 18,758 stars. The 5σ limiting magnitude in y is 21 mag. Aperture photometry was then performed with the IRAF daophot routine with apertures adapted to the FWHM of the frame under the requirement of optimal sampling. Average internal errors are basically photon-noise limited in all bandpasses and reach 1 mmag for stars between 125–135 and 02 at the limiting magnitude in y. These rms values are obtained relative to a sky background in the immediate vicinity of the numerical aperture. External errors are estimated from the rms of all combined nightly frames.

Appendix B: Notes on individual targets

HSS 73 = #1

The CoRoT photometry in imagette modus is overexposed and consequently the relative brightness amplitudes are rather uncertain and are only for orientation. The periodicity of 0.4 d of the light variations should not be affected, but the amplitude is so close to the noise that we are uncertain about its reality and dismiss it. The value in Table 3 is thus very tentative. The spectra are those of a mid A-star with very broad Balmer lines with equivalent widths up to 20 Å for Hα, 14 Å for Hβ, and 10 mÅ for Hγ. Our RVs are only estimates. The diffuse interstellar band (DIB) lines at λ661.3, 578.0 and 579.7 are clearly detected and match those of other cluster members (λ862.1 is not detected).

HSS 314 = #2

Thirtynine RVs confirm this star (also designated Kopff 139) to be a long-period SB1. An orbit was published earlier by Mermilliod et al. (2007) with a period of 3834 ± 36 d (10.5 yrs), an eccentricity of 0.22 and a K-amplitude of just 2.6 km s^-1. Our data span a full amplitude of 4.4 km s^-1 already within 843 d (2.3 yr) and it is thus possible that above orbital period is wrong. However, our data are not sufficient in coverage yet to compute a definite revised orbit but a preliminary circular orbit from STELLA data alone is given in Table 5 with P_orb = 2487 d (6.8 yr). PARSES finds the best fit with T_eff = 5070± 50 K, log g = 3.8 ± 0.1, vsini = 3.9 ± 0.3 km s^-1, and [Fe/H] = −0.16 ± 0.03.

HSS 138 = #3

The Balmer lines appear broad with equivalent widths of 3.5 Å for Hα, 3.0 Å for Hβ, and 3.0 mÅ for Hγ. The interstellar Na D lines are sharp and very prominent. Radial velocities are difficult owing to a lack of spectral lines. The wavelengths with respect to the DIB lines are in agreement with the expected cluster velocity and we are confident that the star is a member. A PARSES solution of the best spectrum suggests T_eff = 6900 K, log g = 3.5, vsini = 100 km s^-1, and [Fe/H] = –0.45 which suggests either a late-A or a slightly evolved mid-F star.

HSS 106 = #4

The star is a single-lined spectroscopic binary with an orbital period of 23 d (Table 5). We sometimes see a double-lined spectrum due to a third star whose velocities appear constant. It is unclear whether this star is a background or confusing source or part of a long-period triple system. No clear photometric period could be extracted from the CoRoT photometry.

HSS 108 = #5

Pulsating star in a single-lined spectroscopic binary. We have 40 RVs that suggest a low-amplitude of just 4 km s^-1 with an orbital period of 25.7 ± 0.15 d. The orbit is so uncertain that we want to mention it only in these notes (other elements are T₀ = 2 456 391, γ = −13.5 ± 1 km s^-1, e = 0, a₁sini = 1.42 ± 0.29 × 10⁶ km, and f(M) = 0.0002 ± 0.0001). The fundamental pulsation period is 0.438 d.

HSS 113 = #6

The Balmer lines appears very broad with equivalent widths of 4 Å for Hα, 2.9 Å for Hβ, and 3 Å for Hγ. The interstellar Na D lines are sharp and prominent. The target is an A–F star with vsini of ≈250 ± 50 km s^-1. The star pulsates with a period of 0.353 d which is likely the fundamental pulsation period in a Delta Scuti-type star. Therefore, RVs are very difficult to obtain and we judge the value of −10 ± 4 km s^-1 to be still in overall agreement with the cluster mean.

HSS 112 = #7

The STELLA RVs do not allow a membership statement. The target has broad Balmer lines suggestive of vsini of ≈150 ± 20 km s^-1. However, no pulsation pattern like in HSS 113 is seen. The periods from the CoRoT light curve are ambiguous, a FFT gives a 14-d value while the ACF suggests ≈40 d.

HSS 107 = #8

Double-lined spectroscopic binary of two nearly equal components. An orbit with P_orb = 11.65 d is given in Table 5. Na D has a strong interstellar absorption component and is partly affected by geocoronal emission. Both components show a Li i λ670.8-nm line with an equivalent width of 80 ± 4 mÅ compared to 110 ± 5 mÅ for the nearby Ca i 671.7 line (both values corrected for two equal continua). Line ratios in the 640-nm region suggest an early-G spectral type and convert the Li equivalent width into an abundance of log n ≈ 2.5 (NLTE) under the assumption of T_eff,1 = T_eff,2 = 5700 K. We measure vsini for both components of 4.5 ± 1.5 km s^-1.

The CoRoT data gave uncertain but consistent photometric periods from a FFT (10 ± 0.5 d) and from the ACF (11 ± 0.5 d). The average is adopted in Table 3.

HSS 356 = #9

Not a cluster member. The SES spectra show it a K5III giant with an average RV of –19.3 km s^-1, almost a perfect match with the M-K standard α Tau. The Balmer lines appear as narrow absorption lines without noticeable wings. Optically thin lines appear also sharp with only small rotational broadening of vsini = 4.5 ± 0.5 km s^-1. No Ca ii H&K nor IRT emission nor Li i 670.8-nm or He i 587.5-nm absorption is detectable. The star is thus not chromospherically active. The red to near-IR part of the spectrum is dominated by molecular lines, e.g., the typical TiO bandheads, e.g., at 705.5 nm. The Na D doublet appears very asymmetric with a broad wing only on the respective blue-wavelength sides but sharp on the red sides. This is due to a stationary emission component redshifted by +43 km s^-1 that is also seen at Hα and K i 769.9 nm where it appears with a redshift of ≈42 km s^-1. The DIB line at λ661.3 nm has an EW of 180 mÅ. The forbidden oxygen lines from [O i] at 557.7, 630.0, and 636.4 nm appear with line intensities relative to the continuum of up to 5, 0.5, and n.d., respectively, and are all mostly of geocoronal origin.

Our PARSES analysis suggests an effective temperature of 3890 ± 50 K and log g of 1.4 ± 0.5. The large error in the metallicity, –0.01 ± 0.22, indicates cross talk in the solution with log g, and thus it is likely that both values are not real. The expected absolute magnitude of a K5 giant is –02 ± 0.2 and its distance modulus is ≈117. Thus, the star is in the cluster background at ≈1.9 kpc.

The CoRoT light curve clearly indicates changing variability with an amplitude of up 015 and a period of 4.25 d. The FFT reveals three discrete peaks at 4.257 d (main peak) 5.253 d and 3.629 d. The FWHM is 0.21 d and the formal ΔΩ/Ω would be 0.782 ± 0.003. The highly variable and modulated light curve and the three isolated periods along with a highly variable ACF could either indicate three different active longitudes or systematic spot evolution. When P is interpreted as the rotational period of the star the minimum radius Rsini would only be 0.38 R_⊙. Furthermore, if we assumed a radius typical for a K5 giant of 25 R_⊙ the inclination would be below 1°. We note that the K5III MK-standard α Tau has a measured radius of 44 R_⊙ (Richichi & Roccatagliata 2005). In any case, it is highly unlikely that the 4.25-d period is the rotation period of the K5 giant but could stem from a very close-by undetected and large-amplitude foreground star.

HSS 241 = #10

Pulsating star with 0.8-d fundamental period. Six RVs, evenly distributed within two months in 2013, show a steady decrease from –11.4 to –25.6 km s^-1. It suggests a SB1-binary with yet unknown systemic velocity. The RV in Table 1 is a current average. The DIB lines at λ661.3, 578.0, and 579.7 are of comparable strength than for cluster members.

HSS 269 = #11

It appears that our STELLA SES data base of HSS 269 consists of three different stars. One target was a plain misidentification (HSS 279; not a cluster member, see below). The other target is a very close neighbor of comparable brightness (HSS 267, separated by 6.7′′). STELLA acquired the correct target in normal weather conditions but the guider may have accidentally picked up the close target during occasions of high wind gusts during at least three occasions. These data were removed but the overall rms of our RVs of HSS 269 remained larger than expected. In addition, CoRoT did not resolve the two targets and thus its photometry is always for both stars.

The STELLA spectrum 20130814B-0006 is for HD 172248 (=HSS 279) and shows a strong He i λ587.5 line with an EW of 160 mÅ. The two Si ii lines at λ634.7 and λ637.1 are measured with EWs of 182 mÅ and 150 mÅ, respectively. The entire spectrum resembles that of a B supergiant. The neutral potassium line K i 769.9 nm is detected with an EW of 40 mÅ and the O i-777 nm triplet is well resolved with a residual intensity of 0.75. However, no Ca ii irt lines are apparent nor do we see the hydrogen Paschen series although a very broad feature spanning the entire echelle order coincides with P12 λ875 nm. PARSES does not converge on a solution, which indicates parameters out of its range (which would be the case for a hot supergiant); vsini appears unmeasurable but the RV is –12.8 km s^-1. The DIB lines at λ661.3, 578.0 and 579.7 are clearly detected with EWs of 98, 125, and 35 mÅ and appear stronger than those of cluster members. We conclude that HSS 279 is not a cluster member but a background early-B supergiant.

In comparison, the other two STELLA spectra obtained on April 8 and June 14, 2013 show a comparable but not identical spectrum to HSS 279, in particular the He i line is completely missing. The two Si ii lines appear weaker with EWs of 100 and 55 mÅ, respectively. The Ca ii irt lines appear weak but are clearly detected, and again no sign of the hydrogen Paschen series. Radial velocities are –23.9 and –20.6 km s^-1 for the two spectra, respectively, which fit the cluster mean, and vsini is ≈13 km s^-1. The DIB lines are detected but are much weaker than in the spectrum of HSS 279 and in agreement with the cluster distance. Hα is typical for a late-F dwarf in agreement with its (b−y)₀ of 0316 from WiFSIP.

The combined CoRoT light curve of HSS 269 and HSS 267 shows a clear and modulated variability with a period of 2.0334 d. If indeed a late-F dwarf, this period is likely its rotation period due to spots.

HSS 208 = #12

Background star with strong DIB lines of 208, 420, 110 mÅ at λ661.3, λ578.0 and λ579.7, respectively. The Balmer and Paschen serie of hydrogen are dominating the spectrum (EW Hα 3.8 Å; EW Hβ 4.2 Å). The ISM Na D lines are saturated and the K i-769.9 line has an EW of 130 mÅ. The spectrum of this star is otherwise almost featureless. The CoRoT photometry shows an oscillation spectrum with a fundamental period of 0.154 d.

HSS 270 = #13

Pulsating hot star with very broad spectral lines. The O i triplet is not resolvable. In contrast, K i 7699/7665 as well as the Na D lines appear very sharp. The Na D lines reach basically zero intensity in the core. If these lines are of ISM origin then the target must be a background star. Then, the average RV of −22.2 ± 2.5 km s^-1 must be a chance agreement with the cluster mean. Unfortunately, this part of the cluster is affected by intervening dust clouds as seen from the WISE images at 4.6 μm and 12 μm and thus the WiFSIP-based de-reddening may be inappropriate. Therefore, we cannot firmly conclude on its membership but removed the target from the isochrone fit.

HSS 57 = #14

The CoRoT data reveal two periods of 10.7 d and ≈1 d that are both seen by naked eye in Fig. 1. Our spectra indicate a He i λ587.5 absorption line with an EW of 120 mÅ. Both Na D lines are dominated by the sharp-lined ISM components. Si ii at λ634.7 and λ637.1 is measured with EWs of 120 mÅ and 50 mÅ, respectively. The He i line and the two Si ii lines indicate a hot star. The rest of the spectrum morphology suggests an early-G or late-F dwarf and the 10.7-d CoRoT period appears to be its rotation period. It is possible that the spectrum is a combination of two stars.

HSS 348 = #15

Double-lined spectroscopic binary with an eclipsing tertiary star.

HSS 95 = #16

The O i triplet at 777 nm appears blended but is clearly detected with a combined EW of 557 mÅ. The traces of Li i 670.7 are very weak due to the low S/N ratio but appears detected on an EW ≈ 50 mÅ level. Other spectrum tracers, like the Hβ, Hγ, Ca ii irt and many of the strong Fe i lines in the blue region, indicate a rapidly-rotating late-F star with vsini = 31 ± 4 km s^-1 in agreement with the 1.68-d CoRoT period. A strong redshifted ISM line with EW of 90 mÅ is seen at K i 769.9 nm (and also at Na i D). The DIB line at λ661.3 has an EW of ≈75 mÅ.

The CoRoT light curve in Fig. 1 clearly indicates period doubling due to two migrating starspots. The full amplitude doubles from 5 mmag to 10 mmag from double-humped to single-humped light curves. The FFT has its main peak at 1.6754 d and a separated peak at 1.7628 d, another period may be real at 1.6096 d but is within the FWHM of 0.0785 d of the main peak.

HSS 174 = #17

Double-lined spectroscopic binary where the weaker component is narrow lined and the stronger component broad lined. The spectrum synthesis picks up only the stronger component and suggests a late-type star with T_eff = 5000 ± 800 K, log g = 4.1 ± 0.5, and a vsini of 14 ± 2 km s^-1. However, its solution is affected by the secondary lines. The metallicity is unconstrained in our fits. Vsini of the secondary is < 4 km s^-1. We measure a Li i 670.8-nm line from the strong-lined component with an equivalent width of 47 ± 3 mÅ (compared to 60 ± 4 mÅ for the nearby Ca i 671.7 line). The spectrum morphology and various line ratios in the 643-nm region suggest two early-G stars with effective temperatures and gravities more near the upper limits of the PARSES results. We assume 5800 K and log g = 4.6 and a continuum ratio of 2:1 (primary to secondary) to convert above Li equivalent width into an abundance.

The FFT shows a single peak at 9.115 d and a smaller peak at 12.14 d. The ACF indicates periods at 9.357 d and 8.2475 d, with the lower period still within the FWHM of 1.86 d of the main peak.

HSS 185 = #19

The spectrum synthesis fits of five spectra consistently give T_eff = 4700 ± 100 K, log g = 3.6 ± 0.3, [Fe/H] = +0.2 ± 0.1, and a vsini of 4 ± 1 km s^-1. Its many sharp photospheric lines, the wing-less Balmer lines, and several line-depth ratios at 643 nm are in agreement with a K0-2 III-IV luminosity classification. The star is therefore not a cluster member. No measurable Li λ670.78 line is present (EW< 10 mÅ).

USNO-B1 0957-0380847 = #20

Not a cluster member. The PARSES effective temperature is 4,630 ± 100 K, log g = 1.5 ± 0.3 and [Fe/H] = −0.45 ± 0.1. Vsini is basically not measurable with a nominal value of 1 ± 1 km s^-1. The interstellar Na i D lines appear very complex and very strong and are split into two broad complexes separated by 1.0 Å, very similar to star #23. The DIB line at λ661.3 has an EW of around 100 mÅ. The Hα profile shows no wings and appears typical for a giant. The star is in the background and is likely a K-type giant. No measurable Li λ670.78 line is present (EW< 10 mÅ).

HSS 194 = #21

Both Hα and Na D show a red-shifted, stationary, and sharp emission-line component. Its redshift is 48 km s^-1 for both D lines and ≈48 km s^-1 for Hα. The D₂ emission appears much stronger (residual intensity of 1.4) than the D₁ emission (residual intensity of around 1.0). Na D appears basically without wings and shows subtle line-profile structure most likely due to intervening interstellar absorption. The forbidden oxygen lines from [O i] at 557.7, 630.0, and 636.4 nm rise above the continuum with relative line intensities of up to 12, 5, and 2, respectively. We believe that all these emission lines are of geo-coronal origin.

The ACF shows a different signal after about seven variability cycles, i.e., after ≈ 23 d, with two distinct maxima of 3.33375 d and 3.51875 d. The two peaks are also visible in the FFT (Fig. 4 and we interpret them as due to two spots co-rotating at two different latitudes). The respective errors are estimated from the FWHM to be 0.13 d.

HSS 158 = #22

Probably not a cluster member. Close binary, likely an eclipsing β Lyrae-type system. Therefore, the period in Table 3 is the orbital period (which is likely also the rotation period because one would expect a bound rotation). The spectrum appears to be that of an A star. Five RVs show a range of –24 km s^-1 to +21 km s^-1. A single spectrum taken on Mar. 30, 2013 shows it with a sharp central hump in the Hα core, most likely due to the RV shift of the two stellar components. All spectra show the Paschen series until P12 at 875.0 nm. Strong DIB lines, e.g., at 6613 Å, also indicate rather a background target than a cluster member.

HSS 89 = #23

Not a cluster member. The PARSES effective temperature is 4560 ± 150 K, log g = 3.3 ± 0.1 and [Fe/H] = −0.02 ± 0.02. Vsini is basically not measurable with an upper limit of 2 km s^-1. The interstellar Na i D lines appear very complex and very strong and are split into two broad complexes. The DIB line at λ661.3 has an EW of around 120 mÅ, but difficult to measure. The star is a background early-K giant. No measurable Li λ670.78 line is present (EW< 10 mÅ).

HSS 198 = #24

Not a cluster member. The spectrum-synthesis fits of five spectra consistently give T_eff = 4 570± 110 K, log g = 2.5± 1.0, [Fe/H] = −0.2± 0.3, and a vsini of 2 ± 1 km s^-1. The star seems to be a background K2 giant or subgiant with strong and complex Na D and K i line profiles. No Li λ670.78 feature greater than 10 mÅ is detectable. The 7-d period from the ACF could be an alias of a longer period. The Lomb-Scargle periodogram gave 7.215 ± 0.03 d with a smaller amplitude than ACF (4 instead of 6 mmag) but an almost equally strong second period at around 16 d.

HSS 209 = #25

Our PARSES converged on three of the five spectra with average values of T_eff = 6060± 240 K, log g = 4.7 ± 0.2, [Fe/H]= −0.2 ± 0.2, and a vsini of 22 ± 4 km s^-1. These parameters suggests an ≈F9 dwarf star. Five RVs outline a weak linear trend from –24.6 km s^-1 in early May 2013 to –23.3 km s^-1 by the end of August 2013 with an unweighted average of −23.9 ± 0.6 km s^-1 (rms). The star could be a long-period binary but the RV evidence is not yet convincing. Strong red-shifted ISM lines at Na D and K i 769.9 nm dominate its profiles. The 3-day period in Table 3 is from a Lomb-Scargle analysis and is very uncertain, the ACF did not give a convincing period at all. Three days is nevertheless in agreement with the measured vsini and the expected radius of an F9 dwarfs and is thus most likely the rotation period of the star.

HSS 259 = #26

Five spectra indicate average values of T_eff = 5200± 400 K, log g = 4.35 ± 0.25, [Fe/H] = −0.9 ± 0.5, and a vsini of 10 ± 2 km s^-1. The star is a rapidly rotating late-G or early-K dwarf. A strong Li i-670.8 line is detected in all of the spectra with an equivalent width of 104 ± 6 mÅ (compared to 100 ± 6 mÅ for the nearby Ca i λ671.7 line). We subtracted an equivalent width of 6 mÅ from above value to obtain the logarithmic abundance of log n ≈ 2.2 in Table 6. The light curve shows occasionally repeated dips that indicate a 5.9-day period, but overall it is not well defined and must be judged preliminary. However, a 5.9-d period is in fair agreement with the vsini and the radius of a late-G dwarf.

HSS 284 = #27

The ACF indicates two close periods at 3.8051 d and 3.8688 d. The FFT periodogram shows only one peak at 3.867 d with a FWHM of 0.3 d. The LSSD revealed two comparable periods of 3.874 d and 4.346 d. Four RVs show a linear change from –22.2 to –24.6 km s^-1 within two months in 2013 indicative of a long-period SB1-binary. The spectrum morphology and two PARSES solutions suggest an effective temperature of 5700 ± 200 K, log g of 4.6 ± 0.1, [Fe/H] = −0.23 ± 0.2 and vsini = 13± 3 km s^-1. The star is a rapidly rotating early-G dwarf. A Li i-670.8 line is detected in one of the spectra with an equivalent width of 46 ± 3 mÅ (compared to 81 ± 4 mÅ for the nearby Ca iλ671.7 line).

HSS 189 = #28

Double-lined spectroscopic binary. Table 5 gives a preliminary orbit based on 8 respective 6 measurements. The period of 110 d is accordingly uncertain. No Li i-670.8 lines detected. The spectrum morphology around the λ643-nm region suggests two mid F-dwarf stars. The ACF period of 7.2 d might be spurious. A Lomb-Scargle periodogram gives three equally strong peaks at 12.15 d, 6.73 d and 5.27 d but with smaller amplitudes than the ACF period.

HSS 211 = #29

An early G-dwarf star. PARSES solutions for five spectra suggest T_eff = 5680 ± 170 K, log g = 4.63 ± 0.03, and [Fe/H] = −0.20 ± 0.06 with vsini of 9 ± 2 km s^-1. A Li i-670.8 line is detected in four of the seven spectra with an average equivalent width of 30 ± 3 mÅ (compared to 100 ± 4 mÅ for the nearby Ca i 671.7 line).

HSS 335 = #30

The target was originally misidentified in our STELLA SES observations and was accidentally observed 26 times. For its RVs, we had chosen a synthetic template spectrum with 6250 K, log g = 4.0 and [Fe/H] = −0.5. The velocities appear to be constant near the cluster mean.

The UBV magnitudes from Herzog et al. (1975) are B = 1359, V = 1292, and B−V = 0.67 and U−B = 0.11 while NOMAD lists magnitudes in BVR of 1346, 1328, and 1252 mag. Herzog et al. (1975) assigned a 90% probability that it is a cluster member. Assuming a 0.8 mag/kpc absorption in the cluster direction, we estimate an absolute magnitude of +46 and a (B−V)₀ of 057, suggestive of an ≈G0 dwarf star. HSS 335 was not detected in the ROSAT HRI survey by Randich et al. (1998) and thus constrains its X-ray luminosity to less then ≈2 × 10²⁹ erg s^-1.

The PARSES solution yields T_eff = 5760 ± 110 K, log g = 4.61 ± 0.05, and [Fe/H] = + 0.05 ± 0.13. The rotational broadening is vsini = 9± 1 km s^-1 with an adopted macroturbulence of 4 km s^-1. The metallicity of HSS 335 is actually consistent with the large range of cluster metallicities in the literature (see Sect. 3.1). The effective temperature and the gravity suggest a solar like (G2) star. Various Fe ii lines are detected. In particular the ratio of the Fe i-643.0 to the Fe ii-643.2 line and the ratio of the Fe ii-645.7 to the Ca i 645.6-line indicate a late-F to early-G spectral class. The luminosity indicator Sr ii 407.7 nm is detectable with an equivalent width of 150 mÅ and appears with half of the strength compared to the nearby Fe i lines 407.1 and 406.3 nm. Its equivalent-width measure is rather uncertain owing to the low S/N but the profile appears unsaturated and thus more compatible with a dwarf luminosity classification than with a giant.

Profiles from Balmer Hα, Hβ, and Hγ appear broad winged with a narrow absorption core, typical for dwarf stars. Their equivalent widths are 5.20 Å, 4.86 Å, and 2.60 Å, respectively; for example, for Hα, 1.3 Å are contained in the core, the rest in the wings. The Ca ii IRT profiles also show a comparable profile with a central absorption core. No Ca ii H&K emission lines are evident. Furthermore, no He i D3 feature is detectable and we can conclude that HSS 335 is at least not a magnetically active star.

We find absorption from the Li i 670.78-nm resonance line. Its measured equivalent width is 43 ± 5 mÅ (approximately half of the nearby Ca i 671.7-nm line which has 90 ± 7 mÅ). The Li line is still blended with a close-by, very weak, Fe i line that cannot be measured in our spectra. Its equivalent width in the Sun (G2V) is 6 mÅ while it is 9 mÅ in a well-exposed spectrum of ϵ Eri (K0V). We assume it to be 6 mÅ for HSS 335 (G0-2) and subtract it from the equivalent-width measure. This translates to a lithium abundance of log n(Li) = 2.2 ± 0.1, based on the usual log n = 12 for hydrogen and an effective temperature of 5760 K.

By far the strongest emission lines are the forbidden oxygen lines from [O i] at 557.7, 630.0, and 636.4 nm with line intensities relative to the continuum of up to 15, 20, and 8, respectively. The line intensities vary by many factors from night to night accompanied by variations of the RV. The changes are perfectly consistent for the three line. No geocoronal emission is seen within the Na i D absorption lines though (like in, e.g., HSS 194, 356, and others), suggesting that the [O i] emission could be partly of interstellar origin. However, we do not see any signs of the low-density sensitive [S ii] or [N ii] emissions at 671.6 nm, 673.1 nm and 654.8 nm, 658.4 nm, respectively, which again suggests that the [O i] emission is of geocoronal origin. Numerous weak and narrow emission lines appear partially with a double-peaked profile, mostly at wavelengths red-ward of 700 nm. The average width at continuum level of these lines is ≈15 km s^-1 (one half of the rotational broadening of the absorption lines). Following Hanuschik (2003), these lines are of telluric origin due to Earth’s airglow layer.

Both photospheric Na i D lines are flanked on the blue side by an additional absorption line of comparable width. The displacement is –15.6 km s^-1 from the cores of the photospheric D lines and their equivalent width is approximately one half of that of the stellar lines. It is generally accepted to be of interstellar origin and related to the column density of sodium in interstellar clouds along the photon path. Hobbs (1974) obtained an empiric distance calibration of the interstellar D₂ equivalent width. If the equivalent width of W_D2 = 180 ± 10 mÅ of HSS 335 is indeed all due to interstellar absorption, then the Hobbs-relation suggests a distance of ≈300 pc. This might still be in agreement with the revised Hipparcos cluster distance of 440 pc.

HSS 150 = #31

Double-lined spectroscopic binary with two comparable components. A synthetic spectrum fit gives vsini = 5± 1 km s^-1, T_eff = 5670± 140 K, log g = 4.5± 0.05, and [Fe/H] = −0.12 ± 0.09 identical to within the error bars for both components. Hα appears never fully resolved in our spectra but the stronger photospheric lines are clearly doubled. The line ratio at λ672 nm is 0.74 ± 0.09 from Ca i 671.7 nm, we designate the stronger-lined component with a and the weaker with b. Both components show a Li i 670.8-nm absorption line as strong as the adjacent Ca i 671.7 line. For the a component, we measure a Li i 670.8-nm line with an equivalent width of 55 ± 3 mÅ (compared to 60 ± 3 mÅ for Ca i 671.7). For the b component the Li EW is 38 ± 3 mÅ (compared to 44 ± 3 mÅ for Ca i). The Li lines appear blended with the close-by (weak) Fe i 670.74-line. The latter amounts to ≈6 mÅ in spectra of the Sun (sky) taken with the same equipment. We subtract this equivalent width from above values and multiply with the respective line ratio and obtain a logarithmic abundance of log n = 2.6 ± 0.2 (85 mÅ) for a and log n = 2.5 ± 0.2 (76 mÅ) for b (NLTE tables of Pavlenko & Magazzú 1996). We note that these values are more uncertain than the others in this paper because the effective temperature is from the composite spectrum.

A FFT of the CoRoT photometry has the highest peak at 12 d, while the ACF peaks at 25 d. We cannot discern which one is the alias but the measured vsini, combined with the assumption of a radius of a G3V star from T_eff and log g, suggests a period of around 10 d.

HSS 63 = #32

Not a cluster member. The mean RV from 6 measurements is –2.6 ± 1.5 km s^-1. The PARSES effective temperature is ≈6000 K and log g ≈ 3.8. vsini is ≈20 km s^-1. Variable [O i] emission lines at 557.7, 630.0, and 636.4 nm are present with line intensities relative to the continuum of 8–60, 1–4, and <1, respectively. The DIB line at λ661.3 appears very strong and has an EW of 130 mÅ. Sodium D lines appear sharp and doubled. Therefore, the star is likely a background target.

HSS 40 = #33

The mean RV from 16 measurements is –12.4 ± 8 km s^-1. Therefore, the stars is formally not a cluster member but the spread of the individual RVs is unusual high. The CoRoT photometry shows no sign for rotational modulation. Hα and Hβ have a shallow core and broad wings and possibly show an asymmetric profile. Variable [O i] emission lines at 557.7, 630.0, and 636.4 nm are present with line intensities relative to the continuum of ≈10, 0.4–4.0, and 0.1–0.8, respectively. The interstellar Na i D lines are very strong and reach basically zero intensity in the line core. The DIB line at λ661.3 appears also very strong and has an EW of 133 mÅ. We conclude that the star is a background target.

HSS 74 = #34

The average RV of –25.7 km s^-1 indicates cluster membership but its rms of 6 km s^-1 indicates it may also be a binary. The PARSES values are only marginally constrained and yield an effective temperature of 5500 ± 400 K, log g = 4.4± 0.7, vsini = 7± 4 km s^-1 and [Fe/H]= −0.18 ± 0.10. We detect a strong Li i λ670.8-nm line with an equivalent width of 70 ± 5 mÅ (compared to 102 ± 5 mÅ for the nearby Ca i 671.7 line). The line is blended with the nearby Fe i 670.74-line and we subtract 6 mÅ from the measured EW based on the solar spectrum. This EW converts to a logarithmic Li abundance of log n = 2.2± 0.1 (NLTE). Hα appears very asymmetric possibly with red-shifted emission.

The CoRoT data show a rotationally modulated light curve with a full amplitude of ≈0025 and a period of 10.1 d. Modulated onto this variation are numerous micro flares. We counted approximately 200 individual events within 78 d with peak amplitudes on the same order of the rotational modulation. This is one of the most extreme flare stars we have ever seen.

HSS 222 = #35

The best exposed of the five spectra show a Li i λ670.8-nm absorption line with an equivalent width of 72 ± 7 mÅ (compared to 130 ± 10 mÅ for the nearby Ca i 671.7 line). The spectrum synthesis allows only an estimate of T_eff = 5770 ± 100 K, log g = 4.52 ± 0.15, [Fe/H]= −0.12 ± 0.04 and vsini = 5 ± 1 km s^-1 but, if taken at face values, convert the de-blended equivalent width (66 mÅ) to a logarithmic NLTE abundance of log n = 2.45 ± 0.1 consistent with other cluster members. The photometric amplitude of ≈20 mmag is the largest of all stars in our sample. The light curve shows a double-humped shape with a period of 9.85 d that we interpret to be the rotation period of the star. With the vsini of 5 km s^-1 the inclination of the rotational axis must be around ≈75°.

HSS 240 = #36

The spectrum synthesis suggests T_eff = 5760 ± 70 K, log g = 4.6 ± 0.1, and a metallicity of –0.10 ± 0.04. Best fit is with vsini = 4 ± 1 km s^-1. Lithium λ670.78 nm is detected with an equivalent width of 90 ± 10 mÅ compared to the Ca i 671.7 line with 102 mÅ. Given the effective temperature of 5760 K from the PARSES fit, the de-blended EW of 84 mÅ converts to a logarithmic Li abundance of log n = 2.6 ± 0.1 (NLTE). The main period from the FFT is at 11.4492 d but is split by a second peak that is visible in the ACF. A formal conversion into ΔΩ/Ω would give 0.421 ± 0.001. A smaller peak in the FFT at 4.92594 d could be produced by a spot group separated by 180 longitudinal degrees from the main group and seen for only a short time. This star shows most likely a high degree of spot evolution instead of differential rotation, explaining the otherwise unusual large value for ΔΩ/Ω (see also target #38).

HSS 165 = #37

We measure a Li i 670.8-nm line with an equivalent width of 83 ± 3 mÅ (compared to 124 ± 10 mÅ for the nearby Ca i 671.7 line). Because of the low vsini of <2.5 km s^-1 the Li line is not blended with the nearby Fe i 670.74-line and thus the effective temperature of 5790 ± 70 K, log g = 4.65± 0.07 and [Fe/H] = −0.03± 0.04 from STELLA/SES convert the equivalent width into a Li abundance of log n = 2.6± 0.1 under NLTE.

USNO-B1 0953-0376947 = #38

The FFT shows large frequency splitting around a period of 8.816 d which would suggest a formal ΔΩ/Ω of 0.72 ± 0.01, nearly four times the solar value. The ACF shows the modulation as well but due the low rotation rate of the star with respect

to the length of the observation, the signal is more likely to be due to spot evolution rather than due to differential rotation.

We measure a Li i λ670.8-nm line with an equivalent width of 80 ± 8 mÅ (compared to 104 ± 5 mÅ for the nearby Ca i 671.7 line). The effective temperature of 5500 K, log g = 4.6 and [Fe/H] = −0.25 ± 0.1 from PARSES suggests a mid-G dwarf consistent with the Hα morphology. Vsini is 7 ± 2 km s^-1. Assuming 6 mÅ contribution from the Fe i blend, we obtain a Li abundance of log n = 2.3± 0.1 (NLTE).

© ESO, 2015