We determine rotation periods of a sample of 48 late F-type to mid-M dwarf stars using time series high-resolution spectroscopy of the Ca ii H&K and H α chromospheric activity indicators. We find ...good agreement between the rotation periods obtained from each of these two indicators. An empirical relationship between the level of chromospheric emission measured by
$\log _{10}(R^{\prime }_{\rm HK}$
) and the spectroscopic rotation periods is reported. This relation is largely independent of the spectral type and the metallicity of the stars and can be used to make a reliable prediction of rotation periods for late K to mid-M dwarfs with low levels of activity. For some stars in the sample, the measured spectroscopic rotation periods coincide, or are very close, to the orbital periods of postulated planets. In such cases, further studies are needed to clarify whether the associated periodic radial velocity signals reveal the existence of planets or are due to magnetic activity.
Aims. We investigate the photometric modulation induced by magnetic activity cycles and study the relationship between rotation period and activity cycle(s) in late-type (FGKM) stars. Methods. We ...analysed light curves, spanning up to nine years, of 125 nearby stars provided by the All Sky Automated Survey (ASAS). The sample is mainly composed of low-activity, main-sequence late-A to mid-M-type stars. We performed a search for short (days) and long-term (years) periodic variations in the photometry. We modelled the light curves with combinations of sinusoids to measure the properties of these periodic signals. To provide a better statistical interpretation of our results, we complement our new results with results from previous similar works. Results. We have been able to measure long-term photometric cycles of 47 stars, out of which 39 have been derived with false alarm probabilities (FAP) of less than 0.1 per cent. Rotational modulation was also detected and rotational periods were measured in 36 stars. For 28 stars we have simultaneous measurements of activity cycles and rotational periods, 17 of which are M-type stars. We measured both photometric amplitudes and periods from sinusoidal fits. The measured cycle periods range from 2 to 14 yr with photometric amplitudes in the range of 5−20 mmag. We found that the distribution of cycle lengths for the different spectral types is similar, as the mean cycle is 9.5 yr for F-type stars, 6.7 yr for G-type stars, 8.5 yr for K-type stars, 6.0 yr for early M-type stars, and 7.1 yr for mid-M-type stars. On the other hand, the distribution of rotation periods is completely different, trending to longer periods for later type stars, from a mean rotation of 8.6 days for F-type stars to 85.4 days in mid-M-type stars. The amplitudes induced by magnetic cycles and rotation show a clear correlation. A trend of photometric amplitudes with rotation period is also outlined in the data. The amplitudes of the photometric variability induced by activity cycles of main-sequence GK stars are lower than those of early- and mid-M dwarfs for a given activity index. Using spectroscopic data, we also provide an update in the empirical relationship between the level of chromospheric activity as given by log10R 'HK and the rotation periods.
ABSTRACT
The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ($ \rm Fe/H \lt -3$), and currently consists of over 4 million ...FGK-type stars over a sky area of $\sim 2500\, \mathrm{deg}^2$. We focus our analysis on a subsample of ∼80 000 main-sequence turn-off stars with heliocentric distances between 6 and 20 kpc, which we take to be a representative sample of the inner halo. The resulting metallicity distribution function (MDF) has a peak at $ \rm Fe/H =-1.6$, and a slope of Δ(LogN)/$\Delta \rm Fe/H = 1.0 \pm 0.1$ in the metallicity range of $-3.4\; \lt\; \rm Fe/H\; \lt -2.5$. This agrees well with a simple closed-box chemical enrichment model in this range, but is shallower than previous spectroscopic MDFs presented in the literature, suggesting that there may be a larger proportion of metal-poor stars in the inner halo than previously reported. We identify the Monoceros/TriAnd/ACS/EBS/A13 structure in metallicity space in a low-latitude field in the anticentre direction, and also discuss the possibility that the inner halo is dominated by a single, large merger event, but cannot strongly support or refute this idea with the current data. Finally, based on the MDF of field stars, we estimate the number of expected metal-poor globular clusters in the Milky Way halo to be 5.4 for $ \rm Fe/H\; \lt\; -2.5$ and 1.5 for $ \rm Fe/H\; \lt\; -3$, suggesting that the lack of low-metallicity globular clusters in the Milky Way is not due simply to statistical undersampling.
Abstract
The Pristine survey is a narrow-band, photometric survey focused around the wavelength region of the Ca ii H&K absorption lines, designed to efficiently search for extremely metal-poor ...stars. In this work, we use the first results of a medium-resolution spectroscopic follow-up to refine the selection criteria for finding extremely metal-poor stars (Fe/H ≤ −3.0) in the Pristine survey. We consider methods by which stars can be selected from available broad-band and infrared photometry plus the additional Pristine narrow-band photometry. The sample consists of 205 stars in the magnitude range 14 < V < 18. Applying the photometric selection criteria cuts the sample down to 149 stars, and from these we report a success rate of 70 per cent for finding stars with Fe/H ≤ −2.5 and 22 per cent for finding stars with Fe/H ≤ −3.0. These statistics compare favourably with other surveys that search for extremely metal-poor stars, namely an improvement by a factor of ∼4 − 5 for recovering stars with Fe/H ≤ −3.0. In addition, Pristine covers a fainter magnitude range than its predecessors and can thus probe deeper into the Galactic halo.
ABSTRACT Our Galaxy is known to contain a central boxy/peanut-shaped bulge, yet the importance of a classical, pressure-supported component within the central part of the Milky Way is still being ...debated. It should be most visible at low metallicity, a regime that has not yet been studied in detail. Using metallicity-sensitive narrow-band photometry, the Pristine Inner Galaxy Survey (PIGS) has collected a large sample of metal-poor ($\rm {Fe/H}\, \lt -1.0$) stars in the inner Galaxy to address this open question. We use PIGS to trace the metal-poor inner Galaxy kinematics as function of metallicity for the first time. We find that the rotational signal decreases with decreasing Fe/H , until it becomes negligible for the most metal-poor stars. Additionally, the velocity dispersion increases with decreasing metallicity for $-3.0 \lt \rm {Fe/H}\, \lt -0.5$, with a gradient of −44 ± 4 km s−1 dex−1. These observations may signal a transition between Galactic components of different metallicities and kinematics, a different mapping on to the boxy/peanut-shaped bulge for former disc stars of different metallicities and/or the secular dynamical and gravitational influence of the bar on the pressure-supported component. Our results provide strong constraints on models that attempt to explain the properties of the inner Galaxy.
Aims. We have identified several tens of extremely metal-poor star candidates from SDSS and LAMOST, which we follow up with the 4.2 m William Herschel Telescope (WHT) telescope to confirm their ...metallicity. Methods. We followed a robust two-step methodology. We first analyzed the SDSS and LAMOST spectra. A first set of stellar parameters was derived from these spectra with the FERRE code, taking advantage of the continuum shape to determine the atmospheric parameters, in particular, the effective temperature. Second, we selected interesting targets for follow-up observations, some of them with very low-quality SDSS or LAMOST data. We then obtained and analyzed higher-quality medium-resolution spectra obtained with the Intermediate dispersion Spectrograph and Imaging System (ISIS) on the WHT to arrive at a second more reliable set of atmospheric parameters. This allowed us to derive the metallicity with accuracy, and we confirm the extremely metal-poor nature in most cases. In this second step we also employed FERRE, but we took a running mean to normalize both the observed and the synthetic spectra, and therefore the final parameters do not rely on having an accurate flux calibration or continuum placement. We have analyzed with the same tools and following the same procedure six well-known metal-poor stars, five of them at Fe/H <−4 to verify our results. This showed that our methodology is able to derive accurate metallicity determinations down to Fe/H <−5.0. Results. The results for these six reference stars give us confidence on the metallicity scale for the rest of the sample. In addition, we present 12 new extremely metal-poor candidates: 2 stars at Fe/H ≃−4, 6 more in the range −4 < Fe / H < −3.5, and 4 more at −3.5 < Fe / H < −3.0. Conclusions. We conclude that we can reliably determine metallicities for extremely metal-poor stars with a precision of 0.2 dex from medium-resolution spectroscopy with our improved methodology. This provides a highly effective way of verifying candidates from lower quality data. Our model spectra and the details of the fitting algorithm are made public to facilitate the standardization of the analysis of spectra from the same or similar instruments.
The biological pump transports organic carbon produced by photosynthesis to the meso- and bathypelagic zones, the latter removing carbon from exchanging with the atmosphere over centennial time ...scales. Organisms living in both zones are supported by a passive flux of particles, and carbon transported to the deep-sea through vertical zooplankton migrations. Here we report globally-coherent positive relationships between zooplankton biomass in the epi-, meso-, and bathypelagic layers and average net primary production (NPP). We do so based on a global assessment of available deep-sea zooplankton biomass data and large-scale estimates of average NPP. The relationships obtained imply that increased NPP leads to enhanced transference of organic carbon to the deep ocean. Estimated remineralization from respiration rates by deep-sea zooplankton requires a minimum supply of 0.44 Pg C y
transported into the bathypelagic ocean, comparable to the passive carbon sequestration. We suggest that the global coupling between NPP and bathypelagic zooplankton biomass must be also supported by an active transport mechanism associated to vertical zooplankton migration.
Context.
It has recently been proposed that LB-1 is a binary system at 4 kpc consisting of a B-type star of 8
M
⊙
and a massive stellar black hole (BH) of 70
M
⊙
. This finding challenges our current ...theories of massive star evolution and formation of BHs at solar metallicity.
Aims.
Our objective is to derive the effective temperature, surface gravity, and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH.
Methods.
We use the non-LTE stellar atmosphere code
FASTWIND
to analyze new and archival high-resolution data.
Results.
We determine (
T
eff
, log
g
) values of (14 000 ± 500 K, 3.50 ± 0.15 dex) that, combined with the
Gaia
parallax, imply a spectroscopic mass, from log
g
, of 3.2
+2.1
−1.9
M
⊙
and an evolutionary mass, assuming single star evolution, of 5.2
+0.3
−0.6
M
⊙
. We determine an upper limit of 8 km s
−1
for the projected rotational velocity and derive the surface abundances; we find the star to have a silicon abundance below solar, and to be significantly enhanced in nitrogen and iron and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and
Gaia
yields similar results for
T
eff
and log
g
and a consistent distance around 2 kpc.
Conclusions.
We propose that the B-type star is a slightly evolved main sequence star of 3–5
M
⊙
with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the
Gaia
parallax. The goodness of fit of the
Gaia
astrometry also favors a high-inclination orbit. If the orbit is edge-on and the B-type star has a mass of 3–5
M
⊙
, the mass of the dark companion would be 4–5
M
⊙
, which would be easier to explain with our current stellar evolutionary models.
Context.
S
TE
P
AR
S
YN
is an automatic code written in Python 3.X designed to infer the stellar atmospheric parameters
T
eff
, log
g
, and Fe/H of FGKM-type stars following the spectral synthesis ...method.
Aims.
We present a description of the S
TE
P
AR
S
YN
code and test its performance against a sample of late-type stars that were observed with the HERMES spectrograph mounted at the 1.2-m
Mercator
Telescope. This sample contains 35 late-type targets with well-known stellar parameters determined independently from spectroscopy. The code is available to the astronomical community in a
GitHub
repository.
Methods.
S
TE
P
AR
S
YN
uses a Markov chain Monte Carlo sampler to explore the parameter space by comparing synthetic model spectra generated on the fly to the observations. The synthetic spectra are generated with an spectral emulator.
Results.
We computed
T
eff
, log
g
, and Fe/H for our sample stars and discussed the performance of the code. We calculated an internal scatter for these targets of −12 ± 117 K in
T
eff
, 0.04 ± 0.14 dex in log
g
, and 0.05 ± 0.09 dex in Fe/H. In addition, we find that the log
g
values obtained with S
TE
P
AR
S
YN
are consistent with the trigonometric surface gravities to the 0.1 dex level. Finally, S
TE
P
AR
S
YN
can compute stellar parameters that are accurate down to 50 K, 0.1 dex, and 0.05 dex for
T
eff
, log
g
, and Fe/H for stars with
v
sin
i
≤ 30 km s
−1
.
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