A Bcool magnetic snapshot survey of solar-type stars

A Bcool magnetic snapshot survey of solar-type stars

Abstract

Stellar magnetic field measurements obtained from spectropolarimetry offer key data for activity and dynamo studies, and we present the results of a major high-resolution spectropolarimetric Bcool project magnetic snapshot survey of 170 solar-type stars from observations with the Telescope Bernard Lyot and the Canada-France-Hawaii Telescope. For each target star a high signal-to-noise circularly polarised Stokes V profile has been obtained using Least-Squares Deconvolution, and used to detect surface magnetic fields and measure the corresponding mean surface longitudinal magnetic field (). Chromospheric activity indicators were also measured.

Surface magnetic fields were detected for 67 stars, with 21 of these stars classified as mature solar-type stars, a result that increases by a factor of four the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic field was detected for 3 out of 18 of the subgiant stars surveyed. For the population of K-dwarfs the mean value of () was also found to be higher (5.7 G) than measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole increases with rotation rate and decreases with age, and the upper envelope for correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies.

This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve understanding of their surface fields and dynamos.

keywords:
line : profiles – Stars : activity – magnetic fields
12

1 Introduction

For the Sun and other slowly-rotating solar-type stars an interface-layer dynamo operating at the tachocline between the radiative and convective zones qualitatively explains the generation of toroidal and poloidal fields, the presence of a solar activity cycle, and the strong dependence of activity on stellar rotation (Parker, 1955; Charbonneau, 2010). Although there remain many aspects of solar magnetic field generation that cannot be comprehensively modelled, or fully understood. For solar-type stars that rotate much faster than the modern-day Sun, the dynamo appears to be generated by a fundamentally different type that operates throughout the whole convection zone, a so-called distributed dynamo whose existence is supported by the observational evidence accumulated throughout the last decade (i.e. Donati et al., 2003; Petit et al., 2004; Marsden et al., 2006), and detailed numerical models (Brown et al., 2010). To better understand dynamos across a range of solar-type stars and their evolution with the star, the observation of a variety of active stars offers an efficient way to probe how various physical stellar parameters (especially the depth of the convective zone or the rotation rate) can enhance or inhibit dynamo processes.

Improved understanding of how the stellar dynamo operates in solar-type stars can be sought by probing the magnetic fields of cool stars across a range of stellar properties. However most techniques used to observe stellar magnetism either involve indirect proxies such as, Ca II H & K, X-ray and radio measurements, or have involved a small number direct measurements obtained from Zeeman broadening (see Reiners, 2012, for a review). Despite these limitations, these surveys have shown a clear rotational dependence and saturation of the stellar dynamo, a relationship between stellar magnetic cycles and stellar rotation period, and a tight correlation between dynamo efficiency and Rossby number (e.g. Saar, 2002; Böhm-Vitense, 2007).

One of the most insightful ways to observe the dynamo in solar-type stars is through the use of spectropolarimetric observations. A significant amount of work has previously been achieved using spectropolarimetric observations of young, active, solar-type stars (i.e. Donati et al., 1997, 2003; Jeffers et al., 2011; Marsden et al., 2011a, b), but it is only with recent advances in instrumentation, such as the development of the ESPaDOnS and NARVAL spectropolarimeters, that it is possible to directly measure solar strength magnetic fields on stars other than the Sun. This approach is complementary to other magnetic field proxies, since polarimetry is providing us with information on the strength and polarity of the large-scale magnetic field component.

The Bcool3 project is an international collaboration of over 70 scientists with a common goal of understanding the magnetic activity of low-mass stars. The Bcool project approaches this question from both theoretical and observational viewpoints and has a number of threads of investigation, ranging from understanding how fully-convective stars generate magnetic fields in the absence of an interface layer (i.e. Donati et al., 2006a; Morin et al., 2008) through to determining if the magnetic field in evolved stars has a fossil or dynamo origin (i.e. Konstantinova-Antova et al., 2012). In this paper we present the first large-scale spectropolarimetric survey of active and inactive solar-type stars to help our understanding of how the magnetic dynamo operates in solar-type main-sequence F, G and K stars as part of the Bcool cool star thread.

There are three main aspects to this Bcool project studies of solar-type stars: (1) a snapshot survey of a large number of solar-type stars, in order to determine the number of stars showing detectable magnetic fields and how the properties of these magnetic fields change with basic stellar parameters (this paper). (2) For a sample of interesting stars from the snapshot survey we have embarked on a program to obtain maps of their global magnetic topology (Petit et al. in prep.). (3) For a further, even smaller, stellar sample we have have undertaken long-term mapping, in order to look for stellar activity cycles (i.e. Petit et al., 2009; Morgenthaler et al., 2012).

The snapshot survey, which is the focus of this paper, has two main aims: (1) to detect magnetic fields on our target sample of stars so that we can select the most suitable targets for long-term mapping of their magnetic field topology (2) to determine if the large-scale magnetic field properties of solar-type stars vary with basic stellar parameters, such as age, temperature, rotation rate and Ca II H & K emission. Additionally, this paper aims to determine if the detection rate of magnetic fields varies significantly with the signal-to-noise ratio of the observations (i.e. is there a minimum signal-to-noise ratio required to detect a magnetic field) and what is the minimum Ca II H & K S-index for which we can get magnetic detections (i.e. is the S-index a good proxy for magnetic field detections).

In this paper we first describe the target selection and the spectropolarimetric observations in Sections 2 and 3. In Sections 4 and 5 we detail the analysis undertaken on the data set and in Section 6 we discuss the results of the survey.

2 The Bcool sample

The goal of the Bcool spectropolarimetric survey is to observe as many of the bright (V 9.0) solar-type stars as possible to further our understanding of the magnetic activity of cool stars. In this first paper we present the spectropolarimetric snapshots of 170 solar-type stars that we have observed starting in 2006 until 2013 as part of the Bcool survey. The initial targets were chosen from Valenti & Fischer (2005) catalogues of “Spectroscpic Properties of Cool Stars I” as they have accurate values for their stellar parameters. Since Valenti & Fischer (2005) mainly focused on G dwarfs, our target list shows a dearth of both F- and K-stars. To address this observational bias, ten additional K stars were included from the Wright et al. (2004) catalogue, and other sources as specified in Table 1, mostly based on their higher activity levels which introduced a bias towards younger K-stars. Additionally, stars that are already known to show magnetic cycles were included from Baliunas et al. (1995) together with a sample of stars of around solar-mass of varying ages to represent the Sun across its evolutionary path (see references in Table 1). Most of the stars in our sample are mature solar-type stars, but there is a large spread in ages (ref Table 1).

HIP SPOCS HD Other Sp. T Log(g) Log(M/H) or Log(Lum) Age Mass Radius Radius v sini Log Refs.
no. no. no. names Type (K) (cm s) Log(Fe/H)* () (Gyr) ( ) () () (km s) (Rossby Number)
400 1 225261 - G9V 5265 4.54 -0.31 -0.341 12.28 0.794 0.80 0.240 0.0 +0.303 1,2
544 4 166 V439 And K0V 5577 4.57 +0.12 -0.215 0.00 0.977 0.88 0.254 4.1 -0.395 1,2
682 6 377 - G2V 5873 4.37 +0.11 0.082 6.12 1.045 1.12 0.308 14.6 -0.395 1,2
1499 13 1461 - G0V 5765 4.37 +0.16 0.078 7.12 1.026 1.11 0.323 1.6 +0.336 1,2
1813 16 1832 - F8 5731 4.28 -0.02 0.119 10.88 0.965 1.18 0.365 2.8 +0.290 1,2
3093 26 3651 54 Psc K0V 5221 4.51 +0.16 -0.286 X 0.882 0.88 0.296 1.1 +0.324 1,2
3203 31 3821 - - 5828 4.51 -0.07 -0.088 0.00 1.011 0.95 0.242 4.3 -0.112 1,2
3206 32 3765 - K2V 5032 4.58 +0.12 -0.465 2.00 0.852 0.79 0.251 0.0 +0.336 1,2
3765 40 4628 - K2.5V 4994 4.59 -0.19 -0.532 X 0.756 0.76 0.238 2.0 +0.294 1,2
3821 41 4614 eta Cas G3V 5941 4.44 -0.17 0.090 2.88 0.991 1.05 0.243 2.8 +0.312 1,2
3979 43 4915 - G6V 5650 4.53 -0.18 -0.164 0.76 0.939 0.88 0.235 1.8 +0.222 1,2
4127 45 5065 - G0 5957 4.08 -0.08 0.460 6.64 1.108 1.60 0.385 4.1 +0.370 1,2
5315 53 6734 29 Cet K0IV 5067 3.76 -0.28 0.753 7.3 2.25 3.10 1.515 2.2 +0.399 1,2
5493 - 6920 4 And F8V 6028 3.78 -0.01* 1.11 X 1.64 3.58 X 11.6 +0.215 21,24
5985 64 7727 40 Cet F8 6022 4.29 +0.03 0.249 5.12 1.101 1.25 0.296 5.0 +0.285 1,2
6405 68 8262 - G3V 5761 4.45 -0.16 -0.035 5.88 0.953 0.98 0.264 1.6 +0.303 1,2
7244 82 9472 - G0 5867 4.52 +0.00 -0.088 0.00 1.037 0.95 0.241 2.2 -0.165 1,2
7276 83 9562 - G1V 5939 4.04 +0.19 0.556 5.04 1.242 1.80 0.452 4.2 +0.390 1,2
7339 84 9407 - G6V 5657 4.42 +0.00 -0.028 8.52 0.951 1.01 0.303 0.1 +0.328 1,2
7513 85 9826 ups And F9V 6213 4.16 +0.12 0.522 3.12 1.310 1.64 0.315 9.6 +0.316 1,2
7585 87 9986 - G5V 5805 4.43 +0.05 0.038 5.08 1.022 1.04 0.284 2.6 +0.275 1,2
7734 90 10086 - G5IV 5725 4.47 +0.09 -0.049 3.76 1.010 0.98 0.273 2.4 +0.093 1,2
7918 - 10307 - G1.5V 5834 4.30 +0.012* 0.135 3.6 1.02 X X 3 +0.332 23,25
7981 95 10476 107 Psc K1V 5181 4.54 -0.07 -0.368 X 0.816 0.82 0.266 1.7 +0.270 1,2
8159 97 10697 109 Psc G5IV 5680 4.03 +0.10 0.448 7.84 1.112 1.73 0.568 2.5 +0.366 1,2
8362 99 10780 V987 Cas K0V 5327 4.52 -0.06 -0.291 10.12 0.846 0.85 0.267 1.3 +0.124 1,2
8486 - 11131 EZ Cet G1Vk: 5805 4.45 -0.061* -0.046 4.9 1.00 X X 4 -0.147 23,25
9349 - 12264 - G5V 5788 4.35 +0.01* 0.007 5.4 1.00 X X 3 +0.114 23,25
9406 109 12328 - G5 4919 3.58 -0.08 0.76 4.04 1.312 3.16 1.997 1.9 +0.422 1,2
9829 112 12846 - G2V 5626 4.42 -0.20 -0.084 11.28 0.877 0.97 0.290 2.2 +0.307 1,2
9911 113 13043 - G2V 5897 4.22 +0.06 0.302 7.16 1.080 1.36 0.363 2.7 +0.324 1,2
10339 119 13531A V451 And G0V 5621 4.54 -0.06 -0.186 0.00 0.957 0.89 0.245 6.0 -0.221 1,2
10505 121 13825 - G8IV 5711 4.38 +0.17 0.053 7.88 1.011 1.09 0.325 1.5 +0.336 1,2
11548 127 15335 13 Tri G0V 5891 3.98 -0.20 0.544 6.76 1.101 1.81 0.454 4.3 +0.370 1,2
12048 128 16141 79 Cet G5IV 5794 4.19 +0.09 0.30 8.68 1.052 1.39 0.416 1.9 +0.362 1,2
12114 129 16160 - K3V 4866 4.62 +0.00 -0.564 0.54 0.809 0.76 0.235 2.9 +0.253 1,2
Table 1: The stellar parameters of the Bcool solar-type star sample. The 5th column gives the spectral type of the star according to SIMBAD. The 13th column gives the radius of the stellar convective zone, while the last two columns give the Log of the Rossby Number (calculated from equation 13 of Wright et al. (2004) from the individual Log(R) measurements given in Table 5) and the list of references (see footnote at the end of the table) where the data has been taken from. : Identifies the stars as a subgiant, see Figure 1 and indicates a possible binary with two stars seen in the Stokes I LSD profile (the analysis has been done on the deeper of the two Stokes I profiles). X indicates an undetermined parameter.
HIP SPOCS HD Other Sp. T Log(g) Log(M/H) or Log(Lum) Age Mass Radius Radius v sini Log Refs.
no. no. no. names Type (K) (cm s) Log(Fe/H)* () (Gyr) ( ) () () (km s) (Rossby Number)
13702 - 18256 rho Ari F6V 6380 4.17 -0.23* 0.82 X 1.56 1.70 X X +0.241 14,20
14150 148 18803 51 Ari G8V 5638 4.44 +0.09 -0.061 7.60 0.962 0.99 0.297 0.8 +0.264 1,2
15457 161 20630 kap Cet G5Vv 5742 4.52 +0.10 -0.084 0.00 1.034 0.95 0.260 5.2 -0.129 1,2
15776 165 21019 - G2V 5529 3.98 -0.36 0.606 7.16 1.654 2.23 0.724 1.7 +0.359 1,2
16537 171 22049 eps Eri K2Vk: 5146 4.61 +0.00 -0.486 0.00 0.856 0.77 0.235 2.4 -0.262 1,2
16641 172 22072 - K1IVe 5027 3.57 -0.19 0.799 2.72 1.396 3.28 1.913 2.4 +0.412 1,2
17027 175 22713 21 Eri K1V 5065 3.68 -0.02 0.699 3.04 1.426 2.91 1.640 2.4 +0.408 1,2
17147 178 22879 - F9V 5688 4.4 -0.76 0.052 X 0.79 1.092 X 1.3 +0.316 1
17183 179 22918 - G5 4939 3.73 +0.01 0.536 6.88 1.161 2.48 1.463 2.4 +0.412 1,2
17378 180 23249 del Eri K1III-IV 5095 4.01 +0.03 0.496 6.28 1.193 2.27 1.189 2.6 +0.399 1,2
18106 187 24213 - G0 6044 4.20 +0.05 0.394 4.28 1.144 1.45 0.325 3.7 +0.362 1,2
18267 189 24496 - G7V 5572 4.52 -0.01 -0.152 3.16 0.956 0.91 0.257 0.0 +0.290 1,2
18606 194 25069 - G9V 4994 3.48 +0.10 0.944 1.72 1.701 3.98 1.999 3.3 +0.449 1,2
19076 199 25680 39 Tau G5V 5874 4.50 +0.04 0.007 0.00 1.071 1.01 0.252 3.8 -0.112 1,2
19849 207 26965 40 Eri K0.5V 5151 4.55 -0.08 -0.380 X 0.808 0.82 0.263 0.5 +0.264 1,2
19925 - 27063 - G0 5767 4.53 +0.071* -0.096 3.0 1.02 X X 2 +0.201 23,25
20800 213 28005 - G0 5819 4.33 +0.29 0.181 5.48 1.103 1.22 0.353 2.4 +0.355 1,2
22319 227 30508 - G5 5206 3.68 -0.21 0.726 3.52 1.326 2.77 1.341 2.6 +0.332 1,2
22336 228 30562 - F8V 5937 4.16 +0.19 0.450 4.00 1.277 1.58 0.390 4.3 +0.366 1,2
22449 230 30652 1 Ori F6V 6424 4.26 +0.00 0.433 2.84 1.236 1.32 0.223 16.8 +0.161 1,2
22633 234 30825 - G5 5176 3.63 -0.21 0.82 2.32 1.443 3.16 1.566 2.5 +0.399 1,2
23311 242 32147 - K3V 4827 4.61 +0.30 -0.551 0.00 0.838 0.78 0.256 1.7 +0.253 1,2
24813 256 34411 15 Aur G1.5IV-V 5911 4.29 +0.09 0.245 6.48 1.081 1.28 0.335 2.0 +0.347 1,2
25278 - 35296 V1119 Tau F8V 6167 4.26 +0.05* 0.26 X 1.22 1.20 X X -0.241 14,21
25486 261 35850 HR 1817 F8V(n)k: 6496 4.37 -0.07 0.231 0.4 1.268 1.22