We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Star Formation in the Milky Way and Nearby Galaxies

The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 ?m and measurements of spectral energy distributions between 52 and 100 ?m at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 ?m array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.

https://iopscience.iop.org/article/10.1086/422717/pdf

The Multiband Imaging Photometer for Spitzer (MIPS)

The Astropy project supports and fosters the development of open-source and openly-developed Python packages that provide commonly-needed functionality to the astronomical community. A key element of the Astropy project is the core package Astropy, which serves as the foundation for more specialized projects and packages. In this article, we provide an overview of the organization of the Astropy project and summarize key features in the core package as of the recent major release, version 2.0. We then describe the project infrastructure designed to facilitate and support development for a broader ecosystem of inter-operable packages. We conclude with a future outlook of planned new features and directions for the broader Astropy project.

The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package

We present the results of a comprehensive assessment of companions to solar-type stars. A sample of 454 stars, including the Sun, was selected from the Hipparcos catalog with {pi}>40 mas, {sigma}{sub {pi}/{pi}} < 0.05, 0.5 {<=} B - V {<=} 1.0 ({approx}F6-K3), and constrained by absolute magnitude and color to exclude evolved stars. These criteria are equivalent to selecting all dwarf and subdwarf stars within 25 pc with V-band flux between 0.1 and 10 times that of the Sun, giving us a physical basis for the term 'solar-type'. New observational aspects of this work include surveys for (1) very close companions with long-baseline interferometry at the Center for High Angular Resolution Astronomy Array, (2) close companions with speckle interferometry, and (3) wide proper-motion companions identified by blinking multi-epoch archival images. In addition, we include the results from extensive radial-velocity monitoring programs and evaluate companion information from various catalogs covering many different techniques. The results presented here include four new common proper-motion companions discovered by blinking archival images. Additionally, the spectroscopic data searched reveal five new stellar companions. Our synthesis of results from many methods and sources results in a thorough evaluation of stellar and brown dwarf companions to nearby Sun-likemore » stars. The overall observed fractions of single, double, triple, and higher-order systems are 56% {+-} 2%, 33% {+-} 2%, 8% {+-} 1%, and 3% {+-} 1%, respectively, counting all confirmed stellar and brown dwarf companions. If all candidate, i.e., unconfirmed, companions identified are found to be real, the percentages would change to 54% {+-} 2%, 34% {+-} 2%, 9% {+-} 2%, and 3% {+-} 1%, respectively. Our completeness analysis indicates that only a few undiscovered companions remain in this well-studied sample, implying that the majority (54% {+-} 2%) of solar-type stars are single, in contrast to the results of prior multiplicity studies. Our sample is large enough to enable a check of the multiplicity dependence on various physical parameters by analyzing appropriate subsamples. Bluer, more massive stars are seen as more likely to have companions than redder, less massive ones, consistent with the trend seen over the entire spectral range. Systems with larger interaction cross sections, i.e., those with more than two components or long orbital periods, are preferentially younger, suggesting that companions may be stripped over time by dynamical interactions. We confirm the planet-metallicity correlation (i.e., higher metallicity stars are more likely to host planets), but are unable to check it for brown dwarfs due to the paucity of such companions, implying that the brown dwarf desert extends over all separation regimes. We find no correlation between stellar companions and metallicity for B - V < 0.625, but among the redder subset, metal-poor stars ([Fe/H] <-0.3) are more likely to have companions with a 2.4{sigma} significance. The orbital-period distribution of companions is unimodal and roughly log normal with a peak and median of about 300 years. The period-eccentricity relation shows the expected circularization for periods below 12 days, caused by tidal forces over the age of the Galaxy, followed by a roughly flat distribution. The mass-ratio distribution shows a preference for like-mass pairs, which occur more frequently in relatively close pairs. The fraction of planet hosts among single, binary, and multiple systems are statistically indistinguishable, suggesting that planets are as likely to form around single stars as they are around components of binary or multiple systems with sufficiently wide separations. This, along with the preference of long orbital periods among stellar systems, increases the space around stars conducive for planet formation, and perhaps life.« less

/pdf/a-survey-of-stellar-families-multiplicity-of-solar-type-4mf1i754rb.pdf

A survey of stellar families: multiplicity of solar-type stars

We present an analysis of the intrinsic colors and temperatures of 5-30 Myr old pre-main sequence (pre-MS) stars using the F0 through M9 type members of nearby, negligibly reddened groups: η Cha cluster, TW Hydra Association, β Pic Moving Group, and Tucana-Horologium Association. To check the consistency of spectral types from the literature, we estimate new spectral types for 52 nearby pre-MS stars with spectral types F3 through M4 using optical spectra taken with the SMARTS 1.5m telescope. Combining these new types with published spectral types, and photometry from the literature (Johnson-Cousins BV IC, 2MASS JHKS and WISE W1, W2, W3, and W4), we derive a new empirical spectral type-color sequence for 5-30 Myr old pre-MS stars. Colors for pre-MS stars match dwarf colors for some spectral types and colors, but for other spectral types and colors, deviations can exceed 0.3 mag. We estimate effective temperatures (Teff) and bolometric corrections (BCs) for our pre-MS star sample through comparing their photometry to synthetic photometry generated using the BT-Settl grid of model atmosphere spectra. We derive a new Teff and BC scale for pre-MS stars, which should be a more appropriate match for T Tauri stars than often-adopted dwarf star scales. While our new Teff scale for pre-MS stars is within ≃100 K of dwarfs at a given spectral type for stars <G5, for G5 through K6, the pre-MS stars are ∼250 K cooler than their main sequence counterparts. Lastly, we present (1) a modern Teff, optical/IR color, and bolometric correction sequence for O9V-M9V MS stars based on an extensive literature survey, (2) a revised Qmethod relation for dereddening UBV photometry of OB-type stars, and (3) introduce two candidate spectral standard stars as representatives of spectral types K8V and K9V. Subject headings: open clusters and associations: individual(η Cha cluster, TW Hydra Association, β Pic Moving Group, Tucana-Horologium Association); — stars: pre-main sequence — stars: fundamental parameters (colors, temperatures)

https://iopscience.iop.org/article/10.1088/0067-0049/208/1/9/pdf

Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars

While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H
and K emission index (R'_(HK) = L_(HK)/L_(bol)) as an empirical age estimator for solar-type dwarfs, existing activity-age relations
produce implausible ages at both high and low activity levels.We have compiled R'_(HK)
HK data from the literature
for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the
cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity
age index,we derive an improved activity-age calibration for F7-K2 dwarfs (0:5 mag < B - V < 0.9 mag). We
also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_(HK) values through
the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that
our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the
Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent
effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal
activity as measured through the fractional X-ray luminosity (R_X = L_X/L_(bol)) has nearly the same age- and rotation inferring
capability as chromospheric activity measured through R'_(HK). As a first application of our calibrations, we
provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within
16 pc.

/pdf/improved-age-estimation-for-solar-type-dwarfs-using-activity-3a3h4zjus7.pdf

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

We present an analysis of the intrinsic colors and temperatures of 5-30 Myr old pre-main sequence (pre-MS) stars using the F0 through M9 type members of nearby, negligibly reddened groups: Eta Cha cluster, TW Hydra Association, Beta Pic Moving Group, and Tucana-Horologium Association. To check the consistency of spectral types from the literature, we estimate new spectral types for 52 nearby pre-MS stars with spectral types F3 through M4 using optical spectra taken with the SMARTS 1.5-m telescope. Combining these new types with published spectral types, and photometry from the literature (Johnson-Cousins BVIc, 2MASS JHKs and WISE W1, W2, W3, and W4), we derive a new empirical spectral type-color sequence for 5-30 Myr old pre-MS stars. Colors for pre-MS stars match dwarf colors for some spectral types and colors, but for other spectral types and colors, deviations can exceed 0.3 mag. We estimate effective temperatures (Teff) and bolometric corrections (BCs) for our pre-MS star sample through comparing their photometry to synthetic photometry generated using the BT-Settl grid of model atmosphere spectra. We derive a new Teff and BC scale for pre-MS stars, which should be a more appropriate match for T Tauri stars than often-adopted dwarf star scales. While our new Teff scale for pre-MS stars is within ~100 K of dwarfs at a given spectral type for stars <G5, for G5 through K6, the pre-MS stars are ~250 K cooler than their main sequence counterparts. Lastly, we present (1) a modern Teff, optical/IR color, and bolometric correction sequence for O9V-M9V MS stars based on an extensive literature survey, (2) a revised Q-method relation for dereddening UBV photometry of OB-type stars, and (3) introduce two candidate spectral standard stars as representatives of spectral types K8V and K9V.

We present a sample of 824 solar and late-type stars with X-ray luminosities and rotation periods. This is used to study the relationship between rotation and stellar activity and derive a new estimate of the convective turnover time. From an unbiased subset of this sample the power-law slope of the unsaturated regime, LX /L bolRo?, is fit as ? = ?2.70 ? 0.13. This is inconsistent with the canonical ? = ?2 slope to a confidence of 5?, and argues for an additional term in the dynamo number equation. From a simple scaling analysis this implies ??/??0.7, i.e., the differential rotation of solar-type stars gradually declines as they spin down. Supersaturation is observed for the fastest rotators in our sample and its parametric dependencies are explored. Significant correlations are found with both the corotation radius and the excess polar updraft, the latter theory providing a stronger dependence and being supported by other observations. We estimate mass-dependent empirical thresholds for saturation and supersaturation and map out three regimes of coronal emission. Late F-type stars are shown never to pass through the saturated regime, passing straight from supersaturated to unsaturated X-ray emission. The theoretical threshold for coronal stripping is shown to be significantly different from the empirical saturation threshold (Ro < 0.13), suggesting it is not responsible. Instead we suggest that a different dynamo configuration is at work in stars with saturated coronal emission. This is supported by a correlation between the empirical saturation threshold and the time when stars transition between convective and interface sequences in rotational spin-down models.

/pdf/the-stellar-activity-rotation-relationship-and-the-evolution-1yp324bzwi.pdf

The stellar activity-rotation relationship and the evolution of stellar dynamos

We present an analysis of the ages and star formation history of the F-type stars in the Upper Scorpius (US), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC) subgroups of Scorpius-Centaurus (Sco-Cen), the nearest OB association. Our parent sample is the kinematically selected Hipparcos sample of de Zeeuw et al., restricted to the 138 F-type members. We have obtained classification-resolution optical spectra and have also determined the spectroscopic accretion disk fraction. With Hipparcos and 2MASS photometry, we estimate the reddening and extinction for each star and place the candidate members on a theoretical H-R diagram. For each subgroup we construct empirical isochrones and compare to published evolutionary tracks. We find that (1) our empirical isochrones are consistent with the previously published age-rank of the Sco-Cen subgroups; (2) subgroups LCC and UCL appear to reach the main-sequence turn-on at spectral types ~F4 and ~F2, respectively. An analysis of the A-type stars shows US reaching the main sequence at about spectral type ~A3. (3) The median ages for the pre-main-sequence members of UCL and LCC are 16 Myr and 17 Myr, respectively, in agreement with previous studies, however we find that (4) Upper Sco is much older than previously thought. The luminosities of the F-type stars in US are typically a factor of ~2.5 less luminous than predicted for a 5 Myr old population for four sets of evolutionary tracks. We re-examine the evolutionary state and isochronal ages for the B-, A-, and G-type Upper Sco members, as well as the evolved M supergiant Antares, and estimate a revised mean age for Upper Sco of 11 ± 1 ± 2 Myr (statistical, systematic). Using radial velocities and Hipparcos parallaxes we calculate a lower limit on the kinematic expansion age for Upper Sco of >10.5 Myr (99% confidence). However, the data are statistically consistent with no expansion. We reevaluate the inferred masses for the known substellar companions in Upper Sco using the revised age and find that the inferred masses are typically ~20%-70% higher than the original estimates which had assumed a much younger age; specifically, we estimate the mass of 1RXS J1609-2105b to be 14+2 –3 M Jup, suggesting that it is a brown dwarf rather than a planet. Finally, we find the fraction of F-type stars exhibiting Hα emission and/or a K-band excess consistent with accretion to be 0/17 (<19%; 95% CL) in US at ~11 Myr, while UCL has 1/41 (2+5 –1%; 68% CL) accretors and LCC has 1/50 (2+4 –1%; 68% CL) accretors at ~16 Myr and ~17 Myr, respectively.

https://iopscience.iop.org/article/10.1088/0004-637X/746/2/154/pdf

Eric E. Mamajek

Papers

Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars

The stellar activity-rotation relationship and the evolution of stellar dynamos

A revised age for upper scorpius and the star formation history among the f-type members of the scorpius-centaurus ob association