Search

We have discovered that the Halpha emission line star Haro 5-2, located in the 3-6 Myr old Ori OB1b association, is a young quadruple system. The system has a 2+2 configuration with an outer separation of 2.6 arcseconds and with resolved subarcsecond inner binary components. The brightest component, Aa, dominates the A-binary, it is a weakline T Tauri star with spectral type M2.5pm1. The two stars of the B component are equally bright at J, but the Bb star is much redder. Optical spectroscopy of the combined B pair indicates a rich emission line spectrum with a M3pm1 spectral type. The spectrum is highly variable and switches back and forth between a classical and a weakline T Tauri star. In the near-infrared, the spectrum shows Paschen beta and Brackett gamma in emission, indicative of active accretion. A significant mid-infrared excess reveals the presence of circumstellar or circumbinary material in the system. Most multiple systems are likely formed during the protostellar phase, involving flybys of neighboring stars followed by an in-spiraling phase driven by accretion from circumbinary material and leading to compact sub-systems. However, Haro 5-2 stands out among young 2+2 quadruples as the two inner binaries are unusually wide relative to the separation of the A and B pair, allowing future studies of the individual components. Assuming the components are coeval, the system could potentially allow stringent tests of PMS evolutionary models., Comment: 21 pages, 10 figures. Accepted by The Astronomical Journal

Pre-main sequence disk accretion is pivotal in determining the final stellar properties and the early conditions for close-in planets. We aim to establish the impact of internal (stellar mass) and external (radiation field) parameters on disk evolution in the Lagoon Nebula massive star-forming region. We employ simultaneous $u,g,r,i,H\alpha$ time series photometry, archival infrared data, and high-precision $K2$ light curves, to derive stellar, disk, and accretion properties for 1012 Lagoon Nebula members. Of all young stars in the Lagoon Nebula, we estimate $34\%-37\%$ have inner disks traceable down to $\sim 12$ $\mu$m, while $38\%-41\%$ are actively accreting. We detect disks $\sim$1.5 times more frequently around G/K/M stars than higher-mass stars, which appear to deplete their inner disks on shorter timescales. We find tentative evidence for faster disk evolution in the central regions of the Lagoon Nebula, where the bulk of the O/B population is located. Conversely, disks appear to last longer at its outskirts, where the measured fraction of disk-bearing stars tends to exceed those of accreting and disk-free stars. The derived mass accretion rates show a non-uniform dependence on stellar mass between $\sim 0.2-5$ $M_\odot$. In addition, the typical accretion rates appear to differ across the Lagoon Nebula extension, with values two times lower in the core region than at its periphery. Finally, we detect tentative density gradients in the accretion shocks, with lags in the appearance of brightness features as a function of wavelength that can amount to $\sim7\%-30\%$ of the rotation period., Comment: 30 pages, 11 figures, two tables; accepted for publication in The Astronomical Journal

We present stellar rotation rates derived from Transiting Exoplanet Survey Satellite (TESS) light curves for stars in Upper Centaurus-Lupus (UCL; ~136 pc, ~16 Myr) and Lower Centaurus-Crux (LCC; ~115 pc, ~17 Myr). We find spot-modulated periods (P) for ~90% of members. The range of light curve and periodogram shapes echoes that found for other clusters with K2, but fewer multi-period stars may be an indication of different noise characteristics of TESS, or a result of the source selection methods here. The distribution of P as a function of color as a proxy for mass fits nicely in between that for both older and younger clusters observed by K2, with fast rotators found among both the highest and lowest masses probed here, and a well-organized distribution of M star rotation rates. About 13% of the stars have an infrared (IR) excess, suggesting a circumstellar disk; this is well-matched to expectations, given the age of the stars. There is an obvious pile-up of disked M stars at P~2 days, and the pile-up may move to shorter P as the mass decreases. There is also a strong concentration of disk-free M stars at P~2 days, hinting that perhaps these stars have recently freed themselves from their disks. Exploring the rotation rates of stars in UCL/LCC has the potential to help us understand the beginning of the end of the influence of disks on rotation, and the timescale on which the star responds to unlocking., Comment: accepted by AAS journals

Using K2, we recently discovered a new type of periodic photometric variability while analysing the light curves of members of Upper Sco (Stauffer \etal\ 2017). The 23 exemplars of this new variability type are all mid-M dwarfs, with short rotation periods. Their phased light curves have one or more broad flux dips or multiple arcuate structures which are not explicable by photospheric spots or eclipses by solid bodies. Now, using TESS data, we have searched for this type of variability in the other major sections of Sco-Cen, Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC). We identify 28 stars with the same light curve morphologies. We find no obvious difference between the Upper Sco and the UCL/LCC representatives of this class in terms of their light curve morphologies, periods or variability amplitudes. The physical mechanism behind this variability is unknown, but as a possible clue we show that the rapidly rotating mid-M dwarfs in UCL/LCC have slightly different colors from the slowly rotating M dwarfs - they either have a blue excess (hot spots?) or a red excess (warm dust?). One of the newly identified stars (TIC242407571) has a very striking light curve morphology. At about every 0.05 in phase are features that resemble icicles, The "icicles" arise because there is a second periodic system whose main feature is a broad flux dip. Using a toy model, we show that the observed light curve morphology results only if the ratio of the two periods and the flux dip width are carefully arranged., Comment: Accepted by AJ

Context. Classical T Tauri stars (cTTs) are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M$_{\odot}$). Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT. Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, r$_{cor}$ $\sim$3.5 R$_{\star}$. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined. Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric accretion process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tau., Comment: 15 pages, 15 figures, accepted in A&A

We present an analysis of K2 light curves (LCs) from Campaigns 4 and 13 for members of the young ($\sim$3 Myr) Taurus association, in addition to an older ($\sim$30 Myr) population of stars that is largely in the foreground of the Taurus molecular clouds. Out of 156 of the highest-confidence Taurus members, we find that 81\% are periodic. Our sample of young foreground stars is biased and incomplete, but nearly all (37/38) are periodic. The overall distribution of rotation rates as a function of color (a proxy for mass) is similar to that found in other clusters: the slowest rotators are among the early M spectral types, with faster rotation towards both earlier FGK and later M types. The relationship between period and color/mass exhibited by older clusters such as the Pleiades is already in place by Taurus age. The foreground population has very few stars, but is consistent with the USco and Pleiades period distributions. As found in other young clusters, stars with disks rotate on average slower, and few with disks are found rotating faster than $\sim$2 d. The overall amplitude of the light curves decreases with age and higher mass stars have generally lower amplitudes than lower mass stars. Stars with disks have on average larger amplitudes than stars without disks, though the physical mechanisms driving the variability and the resulting light curve morphologies are also different between these two classes., Comment: 39 pages, 12 figures. Accepted by AJ

Pre-main sequence stars are variable sources. In diskless stars this variability is mainly due to the rotational modulation of dark photospheric spots and active regions, as in main sequence stars even if associated with a stronger magnetic activity. Aims. We aim at analyzing the simultaneous optical and X-ray variability in these stars to unveil how the activity in the photosphere is connected with that in the corona, to identify the dominant surface magnetic activity, and to correlate our results with stellar properties, such as rotation and mass. Methods. We analyzed the simultaneous optical and X-ray variability in stars without inner disks (e.g., class III objects and stars with transition disks) in NGC 2264 from observations obtained with Chandra/ACIS-I and CoRoT as part of the Coordinated Synoptic Investigation of NGC 2264. We searched for those stars whose optical and X-ray variability is correlated, anti-correlated, or not correlated by sampling their optical and X-ray light curves in suitable time intervals and studying the correlation between the flux observed in optical and in X-rays. We then studied how this classification is related with stellar properties. Results. Starting from a sample of 74 class III/transition disk (TD) stars observed with CoRoT and detected with Chandra with more than 60 counts, we selected 16 stars whose optical and X-ray variability is anti-correlated, 11 correlated, and 17 where there is no correlation. The remaining stars did not fall in any of these groups. We interpreted the anti-correlated optical and X-ray variability as typical of spot-dominated sources, due to the rotational modulation of photospheric spots spatially coincident to coronal active regions, and correlated variability typical of faculae-dominated sources, where the brightening due to faculae is dominant over the darkening due to spots. [Conclusions not shown in the pre-print], Comment: Accepted for publication on Astronomy & Astrophysics. Appendices are not included in this preprint

We present a catalog of 417 luminous infrared variable stars with periods exceeding 250 days. These were identified in 20 nearby galaxies by the ongoing SPIRITS survey with the Spitzer Space Telescope. Of these, 359 variables have $M_{[4.5]}$ (phase-weighted mean magnitudes) fainter than $-12$ and periods and luminosities consistent with previously reported variables in the Large Magellanic Cloud. However, 58 variables are more luminous than $M_{[4.5]} = -12$, including 11 that are brighter than $M_{[4.5]} = -13$ with the brightest having $M_{[4.5]} = -15.51$. Most of these bright variable sources have quasi-periods longer than 1000 days, including four over 2000 days. We suggest that the fundamental period-luminosity relationship, previously measured for the Large Magellanic Cloud, extends to much higher luminosities and longer periods in this large galaxy sample. We posit that these variables include massive AGB stars (possibly super-AGB stars), red supergiants experiencing exceptionally high mass-loss rates, and interacting binaries. We also present 3.6, 4.5, 5.8 and 8.0 $\mu$m photometric catalogs for all sources in these 20 galaxies., Comment: 18 pages, 25 figures

We present an exquisite, 30-min cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Pan-STARRS1 and CTIO 4-m DECam observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical SNe Ia. This "flux excess" relative to canonical SN Ia behavior is confirmed in our $i$-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14$\pm0.04$ days after explosion, has a FWHM of 3.12$\pm0.04$ days, a blackbody temperature of $T=17,500^{+11,500}_{-9,000}$ K, a peak luminosity of $4.3\pm0.2\times10^{37}\,{\rm erg\,s^{-1}}$, and a total integrated energy of $1.27\pm0.01\times10^{43}\,{\rm erg}$. We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of $\sim$$2\times10^{12}\,{\rm cm}$ based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system., Comment: 20 pages, 9 figures, 2 tables. Submitted to APJ Letters on 31 Jul 2018, Accepted for publication on 31 Aug 2018

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically-confirmed type Ia supernova (SN Ia) observed in the $Kepler$ field. The $Kepler$ data revealed an excess emission in its early light curve, allowing to place interesting constraints on its progenitor system (Dimitriadis et al. 2018, Shappee et al. 2018b). Here, we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3$\pm$0.3 days and $\Delta$m$_{15}(B)=0.96\pm$0.03 mag, but it seems to have bluer $B - V$ colors. We construct the "uvoir" bolometric light curve having peak luminosity as 1.49$\times$10$^{43}$erg s$^{-1}$, from which we derive a nickel mass as 0.55$\pm$0.04M$_{\odot}$ by fitting radiation diffusion models powered by centrally located $^{56}$Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of $^{56}$Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a non-degenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia, but is characterized by prominent and persistent carbon absorption features. The C II features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in a SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers., Comment: 48 pages, 23 figures. This paper is part of a coordinated effort between groups. Accepted for publication in ApJ

Magnetospheric accretion has been thoroughly studied in young stellar systems with full non-evolved accretion disks, but it is poorly documented for transition disk objects with large inner cavities. We aim at characterizing the star-disk interaction and the accretion process onto the central star of LkCa 15, a transition disk system with an inner dust cavity. We obtained quasi-simultaneous photometric and spectropolarimetric observations of the system over several rotational periods. We analyzed the system light curve, as well as changes in spectral continuum and line profile to derive the properties of the accretion flow from the edge of the inner disk to the central star. We also derived magnetic field measurements at the stellar surface. We find that the system exhibits magnetic, photometric, and spectroscopic variability with a period of about 5.70 days. The light curve reveals a periodic dip, which suggests the presence of an inner disk warp that is located at the corotation radius at about 0.06 au from the star. Line profile variations and veiling variability are consistent with a magnetospheric accretion model where the funnel flows reach the star at high latitudes. This leads to the development of an accretion shock close to the magnetic poles. All diagnostics point to a highly inclined inner disk that interacts with the stellar magnetosphere. The spectroscopic and photometric variability of LkCa 15 is remarkably similar to that of AA Tau, the prototype of periodic dippers. We therefore suggest that the origin of the variability is a rotating disk warp that is located at the inner edge of a highly inclined disk close to the star. This contrasts with the moderate inclination of the outer transition disk seen on the large scale and thus provides evidence for a significant misalignment between the inner and outer disks of this planet-forming transition disk system., Comment: 16 pages, 22 figures, accepted by Astronomy & Astrophysics

On 2018 Feb. 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z=0.01098 and a peak apparent magnitude of B_{max}=14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and pre-discovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System (ATLAS). The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN~Ia light curve, and it unambiguously shows a ~4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a non-degenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing, published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe~Ia light curves. Finally, we use Swift X-ray non-detections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant density CSM these non-detections constrain rho<4.5 * 10^5 cm^-3 at a radius of 4 *10^15 cm from the progenitor star. Assuming a wind-like environment, we place mass-loss limits of Mdot< 8 * 10^-6 M_sun yr^-1 for v_w=100 km s^-1, ruling out some symbiotic progenitor systems., Comment: 17 pages, 8 figures, 3 Tables. Accepted to ApJ. This work is part of a number of papers analyzing ASASSN-18bt, with coordinated papers from Dimitriadis et al. (2018) and Li et al. (2018)

Flares from the Sun and other stars are most prominently observed in the soft X-ray band. Most of the radiated energy, however, is released at optical/UV wavelengths. In spite of decades of investigation, the physics of flares is not fully understood. Even less is known about the powerful flares routinely observed from pre-main sequence stars, which might significantly influence the evolution of circumstellar disks. Observations of the NGC2264 star forming region were obtained in Dec. 2011, simultaneously with three telescopes, Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the "Coordinated Synoptic Investigation of NGC2264" (CSI-NGC2264). Shorter Chandra and CoRoT observations were also obtained in March 2008. We analyzed the lightcurves to detect X-ray flares with an optical and/or mIR counterpart. Basic flare properties from the three datasets, such as emitted energies and peak luminosities, were then compared to constrain the spectral energy distribution of the flaring emission and the physical conditions of the emitting regions. Flares from stars with and without circumstellar disks were also compared to establish any difference that might be attributed to the presence of disks. Seventy-eight X-ray flares with an optical and/or mIR counterpart were detected. Their optical emission is found to correlate well with, and to be significantly larger than, the X-ray emission. The slopes of the correlations suggest that the difference becomes smaller for the most powerful flares. The mIR flare emission seems to be strongly affected by the presence of a circumstellar disk: flares from stars with disks have a stronger mIR emission with respect to stars without disks. This might be attributed to the reprocessing of the optical (and X-ray) flare emission by the inner circumstellar disk, providing evidence for flare-induced disk heating., Comment: 16 pages (36 including appendixes), 8 figures (main text), accepted for publication by Astronomy & Astrophysics (section 8)

We present an analysis of K2 light curves (LCs) for candidate members of the young Upper Sco (USco) association $\sim$8 Myr) and the neighboring Rho Oph embedded cluster ($\sim$1 Myr). We establish $\sim$1300 stars as probable members, $\sim$80\% of which are periodic. The phased LCs have a variety of shapes which can be attributed to physical causes ranging from stellar pulsation and stellar rotation to disk-related phenomena. We identify and discuss a number of observed behaviors. The periods are $\sim$0.2-30 days with a peak near 2 days and the rapid period end nearing break-up velocity. M stars in the young USco region rotate systematically faster than GK stars, a pattern also present in K2 data for the older Pleiades and Praesepe systems. At higher masses (types FGK), the well-defined period-color relationship for slowly rotating stars seen in the Pleiades and Praesepe is not yet present in USco. Circumstellar disks are present predominantly among the more slowly rotating Ms in USco, with few disks in the sub-day rotators. However, M dwarfs with disks rotate faster on average than FGK systems with disks. For four of these disked Ms, we provide direct evidence for disk-locking based on the K2 LC morphologies. Our preliminary analysis shows a relatively mass-independent spin-up by a factor of $\sim$3.5 between USco and the Pleiades, then mass-dependent spin-down between Pleiades and Praesepe., Comment: Accepted by AJ

We explore the structure and star formation history of the open cluster NGC 2264 (~3 Myr). We combined spectroscopic data from the Gaia-ESO Survey (GES) with multi-wavelength photometry from the Coordinated Synoptic Investigation of NGC 2264 (CSI 2264). We examined a sample of 655 cluster members, including both disk-bearing and disk-free young stars. We find a significant age spread of 4-5 Myr among cluster members. Disk-bearing objects are statistically associated with younger isochronal ages than disk-free sources. The cluster has a hierarchical structure, with two main blocks. The northern half develops around the O-type binary star S Mon; the southern half, close to the tip of the Cone Nebula, contains the most embedded regions of NGC 2264, populated mainly by objects with disks and ongoing accretion. The median ages of objects at different locations within the cluster, and the spatial distribution of disked and non-disked sources, suggest that star formation began in the north of the cluster, over 5 Myr ago, and was ignited in its southern region a few Myr later. Star formation is likely still ongoing in the most embedded regions of the cluster, while the outer regions host a widespread population of more evolved objects. We find a detectable lag between the typical age of disk-bearing objects and that of accreting objects in the inner regions of NGC 2264: the first tend to be older than the second, but younger than disk-free sources at similar locations within the cluster. This supports earlier findings that the characteristic timescales of disk accretion are shorter than those of disk dispersal, and smaller than the average age of NGC 2264 (i.e., < 3 Myr). At the same time, disks in the north of the cluster tend to be shorter-lived (~2.5 Myr) than elsewhere; this may reflect the impact of massive stars within the region (notably S Mon), that trigger rapid disk dispersal., Comment: 24 pages, 14 figures, 7 tables; accepted for publication in A&A. Abstract shortened for arXiv listing

We have Fourier analyzed 941 K2 light curves of likely members of Praesepe, measuring periods for 86% and increasing the number of rotation periods (P) by nearly a factor of four. The distribution of P vs. (V-K), a mass proxy, has three different regimes: (V-K)<1.3, where the rotation rate rapidly slows as mass decreases; 1.3<(V-K)<4.5, where the rotation rate slows more gradually as mass decreases; and (V-K)>4.5, where the rotation rate rapidly increases as mass decreases. In this last regime, there is a bimodal distribution of periods, with few between $\sim$2 and $\sim$10 days. We interpret this to mean that once M stars start to slow down, they do so rapidly. The K2 period-color distribution in Praesepe ($\sim$790 Myr) is much different than in the Pleiades ($\sim$125 Myr) for late F, G, K, and early-M stars; the overall distribution moves to longer periods, and is better described by 2 line segments. For mid-M stars, the relationship has similarly broad scatter, and is steeper in Praesepe. The diversity of lightcurves and of periodogram types is similar in the two clusters; about a quarter of the periodic stars in both clusters have multiple significant periods. Multi-periodic stars dominate among the higher masses, starting at a bluer color in Praesepe ((V-K)$\sim$1.5) than in the Pleiades ((V-K)$\sim$2.6). In Praesepe, there are relatively more light curves that have two widely separated periods, $\Delta P >$6 days. Some of these could be examples of M star binaries where one star has spun down but the other has not., Comment: Accepted by ApJ

Pre-main sequence stars are variable sources. In stars with disks, this variability is related to the morphology of the inner circumstellar region (<0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during optical bursts and dips in order to unveil the nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of optical extinction and X-ray absorption during the optical dips; in stars with accretion bursts, we search for soft X-ray emission and increasing X-ray absorption during the bursts. Results. We find evidence for coherent optical and X-ray flux variability among the stars with variable extinction. In 9/24 stars with optical dips, we observe a simultaneous increase of X-ray absorption and optical extinction. In seven dips, it is possible to calculate the NH/AV ratio in order to infer the composition of the obscuring material. In 5/20 stars with optical accretion bursts, we observe increasing soft X-ray emission during the bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and bursts, since favorable geometric configurations are required. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion bursts, we observe on average a larger soft X-ray spectral component not observed in non accreting stars., Comment: Accepted for publication by Astronomy & Astrophysics

The low spin rates measured for solar-type stars at an age of a few Myr (~10% of the break-up velocity) indicate that some mechanism of angular momentum regulation must be at play in the early pre-main sequence. We characterize the rotation properties for members of the region NGC 2264 (~3 Myr), and investigate the accretion-rotation connection at an age where about 50% of the stars have already lost their disks. We examined a sample of 500 cluster members whose photometric variations were monitored in the optical for 38 consecutive days with CoRoT. Light curves were analyzed for periodicity using the Lomb-Scargle periodogram, the autocorrelation function and the string-length method. The period distribution obtained for the cluster consists of a smooth distribution centered around P=5.2 d with two peaks at P=1-2 d and 3-4 d. A separate analysis of CTTS and WTTS indicates that the P=1-2 d peak is associated with the latter, while both groups contribute to the P=3-4 d peak. The comparison between CTTS and WTTS supports the idea of a rotation-accretion connection: their respective rotational properties are statistically different, and CTTS rotate on average more slowly than WTTS. We also observe a clear dearth of fast rotators with strong accretion signatures (large UV flux excess). This is consistent with earlier findings that fast rotators in young star clusters are typically devoid of dusty disks. Our sample shows some evidence of a mass dependence in the rotation properties of NGC 2264 members, lower-mass stars spinning on average faster. This study confirms that disks influence the rotational evolution of young stars. The idea of disk-locking may be consistent with the picture of rotation and rotation-accretion connection that we observe for the NGC 2264 cluster. However, the origin of the several substructures that we observe in the period distribution deserves further investigation., Comment: 45 pages (27 pages for the main text, plus data table and 5 Appendices); 4 tables, 24 figures. Accepted for publication in Astronomy & Astrophysics. Abstract shortened for the arXiv listing

Solicited public opinion surveys reach a limited subpopulation of willing participants and are expensive to conduct, leading to poor time resolution and a restricted pool of expert-chosen survey topics. In this study, we demonstrate that unsolicited public opinion polling through sentiment analysis applied to Twitter correlates well with a range of traditional measures, and has predictive power for issues of global importance. We also examine Twitter's potential to canvas topics seldom surveyed, including ideas, personal feelings, and perceptions of commercial enterprises. Two of our major observations are that appropriately filtered Twitter sentiment (1) predicts President Obama's job approval three months in advance, and (2) correlates well with surveyed consumer sentiment. To make possible a full examination of our work and to enable others' research, we make public over 10,000 data sets, each a seven-year series of daily word counts for tweets containing a frequently used search term.

We present the results of optical, near-infrared, and mid-infrared observations of M101 OT2015-1 (PSN J14021678+5426205), a luminous red transient in the Pinwheel galaxy (M101), spanning a total of 16 years. The lightcurve showed two distinct peaks with absolute magnitudes $M_r\leq-12.4$ and $M_r \simeq-12$, on 2014 November 11 and 2015 February 17, respectively. The spectral energy distributions during the second maximum show a cool outburst temperature of $\approx$3700 K and low expansion velocities ($\approx-$300 \kms) for the H I, Ca II, Ba II and K I lines. From archival data spanning 15 to 8 years before the outburst, we find a single source consistent with the optically discovered transient which we attribute to being the progenitor; it has properties consistent with being an F-type yellow supergiant with $L$~$\sim$~8.7~$\times\ 10^4$ \Lsun, $T_{\rm{eff}}\approx$7000~K and an estimated mass of $\rm{M1}= 18\pm 1$ \Msun. This star has likely just finished the H burning phase in the core, started expanding, and is now crossing the Hertzsprung gap. Based on the combination of observed properties, we argue that the progenitor is a binary system, with the more evolved system overfilling the Roche lobe. Comparison with binary evolution models suggests that the outburst was an extremely rare phenomenon, likely associated with the ejection of the common envelope. The initial mass of the binary progenitor system fills the gap between the merger candidates V838 Mon (5$-$10 \Msun) and NGC~4490-OT~(30~\Msun)., Comment: 19 pages, 10 figures, 6 tables. Accepted by ApJ

Young (125 Myr), populous ($>$1000 members), and relatively nearby, the Pleiades has provided an anchor for stellar angular momentum models for both younger and older stars. We used K2 to explore the distribution of rotation periods in the Pleiades. With more than 500 new periods for Pleiades members, we are vastly expanding the number of Pleiads with periods, particularly at the low mass end. About 92\% of the members in our sample have at least one measured spot-modulated rotation period. For the $\sim$8\% of the members without periods, non-astrophysical effects often dominate (saturation, etc.), such that periodic signals might have been detectable, all other things being equal. We now have an unusually complete view of the rotation distribution in the Pleiades. The relationship between $P$ and $(V-K_{\rm s})_0$ follows the overall trends found in other Pleiades studies. There is a slowly rotating sequence for $1.1\lesssim(V-K_{\rm s})_0\lesssim 3.7$, and a primarily rapidly rotating population for $(V-K_{\rm s})_0\gtrsim 5.0$. There is a region in which there seems to be a disorganized relationship between $P$ and $(V-K_{\rm s})_0$ for $3.7 \lesssim(V-K_{\rm s})_0\lesssim 5.0$. Paper II continues the discussion, focusing on multi-period structures, and Paper III speculates about the origin and evolution of the period distribution in the Pleiades., Comment: 26 pages, 15 figures; electronic-only tables and figures available upon request to the author. Accepted by AJ

We use high quality K2 light curves for hundreds of stars in the Pleiades to understand better the angular momentum evolution and magnetic dynamos of young, low mass stars. The K2 light curves provide not only rotational periods but also detailed information from the shape of the phased light curve not available in previous studies. A slowly rotating sequence begins at $(V-K_{\rm s})_0\sim$1.1 (spectral type F5) and ends at $(V-K_{\rm s})_0\sim$ 3.7 (spectral type K8), with periods rising from $\sim$2 to $\sim$11 days in that interval. Fifty-two percent of the Pleiades members in that color interval have periods within 30\% of a curve defining the slow sequence; the slowly rotating fraction decreases significantly redward of $(V-K_{\rm s})_0$=2.6. Nearly all of the slow-sequence stars show light curves that evolve significantly on timescales less than the K2 campaign duration. The majority of the FGK Pleiades members identified as photometric binaries are relatively rapidly rotating, perhaps because binarity inhibits star-disk angular momentum loss mechanisms during pre-main sequence evolution. The fully convective, late M dwarf Pleiades members (5.0 $<(V-K_{\rm s})_0<$ 6.0) nearly always show stable light curves, with little spot evolution or evidence of differential rotation. During PMS evolution from $\sim$3 Myr (NGC2264 age) to $\sim$125 Myr (Pleiades age), stars of 0.3 $M_{\odot}$ shed about half their angular momentum, with the fractional change in period between 3 and 125 Myr being nearly independent of mass for fully convective stars. Our data also suggest that very low mass binaries form with rotation periods more similar to each other and faster than would be true if drawn at random from the parent population of single stars., Comment: 30 pages, 26 figures, accepted to AJ. Paper I is arXiv:1606.00052 and Paper II is arXiv:1606.00055

We use K2 to continue the exploration of the distribution of rotation periods in Pleiades that we began in Paper I. We have discovered complicated multi-period behavior in Pleiades stars using these K2 data, and we have grouped them into categories, which are the focal part of this paper. About 24% of the sample has multiple, real frequencies in the periodogram, sometimes manifesting as obvious beating in the light curves. Those having complex and/or structured periodogram peaks, unresolved multiple periods, and resolved close multiple periods are likely due to spot/spot group evolution and/or latitudinal differential rotation; these largely compose the slowly rotating sequence in $P$ vs.~$(V-K_{\rm s})_0$ identified in Paper I. The fast sequence in $P$ vs.~$(V-K_{\rm s})_0$ is dominated by single-period stars; these are likely to be rotating as solid bodies. Paper III continues the discussion, speculating about the origin and evolution of the period distribution in the Pleiades., Comment: 28 pages, 15 figures. Accepted by AJ

We investigate here the lithium content of young low-mass stars in the 5~Myr-old star forming region NGC~2264 and its relationship with rotation. We combine lithium equivalent width measurements, EW(Li), from the Gaia-ESO Survey with the determination of rotational periods from the CSI~2264 survey. We consider only bona fide non accreting cluster members in order to minimize uncertainties on EW(Li). We report the existence of a relationship between lithium content and rotation in NGC~2264 at an age of 5~Myr. The Li-rotation connection is seen over a restricted temperature range, Teff=3800--4400K, where fast rotators are Li-rich compared to slow ones. This correlation is similar to, albeit of lower amplitude than, the Li-rotation connection previously reported for K dwarfs in the 125 Myr-old Pleiades cluster. We investigate whether the non-standard pre-main sequence models developed so far to explain the Pleiades results, which are based on episodic accretion, pre-main sequence core-envelope decoupling, and/or radius inflation due to enhanced magnetic activity, can account for an early development of the Li-rotation connection. While radius inflation appears to be the most promising possibility, each of these models has issues. We therefore also discuss external causes that might operate during the first few Myr of pre-main sequence evolution, such as planet engulfment and/or steady disk accretion, as possible candidates for the common origin for Li-excess and fast rotation in young low-mass pre-main sequence stars. The emergence of a connection between lithium content and rotation rate at such an early age as 5~Myr suggests a complex link between accretion processes, early angular momentum evolution, and possibly planet formation, which likely impacts early stellar evolution and still is to be fully deciphered., Comment: 9 pages, 5 figures, Astronomy & Astrophysics, in press

Although climate change and energy are intricately linked, their explicit connection is not always prominent in public discourse and the media. Disruptive extreme weather events, including hurricanes, focus public attention in new and different ways, offering a unique window of opportunity to analyze how a focusing event influences public discourse. Media coverage of extreme weather events simultaneously shapes and reflects public discourse on climate issues. Here we analyze climate and energy newspaper coverage of Hurricanes Katrina (2005) and Sandy (2012) using topic models, mathematical techniques used to discover abstract topics within a set of documents. Our results demonstrate that post-Katrina media coverage does not contain a climate change topic, and the energy topic is limited to discussion of energy prices, markets, and the economy with almost no explicit linkages made between energy and climate change. In contrast, post-Sandy media coverage does contain a prominent climate change topic, a distinct energy topic, as well as integrated representation of climate change and energy, indicating a shift in climate and energy reporting between Hurricane Katrina and Hurricane Sandy.

As part of the Young Stellar Object VARiability (YSOVAR) program, we monitored NGC 1333 for ~35 days at 3.6 and 4.5 um using the Spitzer Space Telescope. We report here on the mid-infrared variability of the point sources in the ~10x~20arcmin area centered on 03:29:06, +31:19:30 (J2000). Out of 701 light curves in either channel, we find 78 variables over the YSOVAR campaign. About half of the members are variable. The variable fraction for the most embedded SEDs (Class I, flat) is higher than that for less embedded SEDs (Class II), which is in turn higher than the star-like SEDs (Class III). A few objects have amplitudes (10-90th percentile brightness) in [3.6] or [4.5]>0.2 mag; a more typical amplitude is 0.1-0.15 mag. The largest color change is >0.2 mag. There are 24 periodic objects, with 40% of them being flat SED class. This may mean that the periodic signal is primarily from the disk, not the photosphere, in those cases. We find 9 variables likely to be 'dippers', where texture in the disk occults the central star, and 11 likely to be 'bursters', where accretion instabilities create brightness bursts. There are 39 objects that have significant trends in [3.6]-[4.5] color over the campaign, about evenly divided between redder-when-fainter (consistent with extinction variations) and bluer-when-fainter. About a third of the 17 Class 0 and/or jet-driving sources from the literature are variable over the YSOVAR campaign, and a larger fraction (~half) are variable between the YSOVAR campaign and the cryogenic-era Spitzer observations (6-7 years), perhaps because it takes time for the envelope to respond to changes in the central source. The NGC 1333 brown dwarfs do not stand out from the stellar light curves in any way except there is a much larger fraction of periodic objects (~60% of variable brown dwarfs are periodic, compared to ~30% of the variables overall)., Comment: 84 pages, 43 figures. Accepted by AJ

We present an IR-monitoring survey with the $Spitzer$ Space Telescope of the star forming region GGD 12-15. Over 1000 objects were monitored including about 350 objects within the central 5 arcminutes which is found to be especially dense in cluster members. The monitoring took place over 38 days and is part of the Young Stellar Object VARiability (YSOVAR) project. The region was also the subject of a contemporaneous 67ks $Chandra$ observation. The field includes 119 previously identified pre-main sequence star candidates. X-rays are detected from 164 objects, 90 of which are identified with cluster members. Overall, we find that about half the objects in the central 5 arcminutes are young stellar objects based on a combination of their spectral energy distribution, IR variability and X-ray emission. Most of the stars with IR excess relative to a photosphere show large amplitude (>0.1 mag) mid-IR variability. There are 39 periodic sources, all but one of these is found to be a cluster member. Almost half of the periodic sources do not show IR excesses. Overall, more than 85% of the Class I, flat spectrum, and Class II sources are found to vary. The amplitude of the variability is larger in more embedded young stellar objects. Most of the Class~I/II objects exhibit redder colors in a fainter state, compatible with time-variable extinction. A few become bluer when fainter, which can be explained with significant changes in the structure of the inner disk. A search for changes in the IR due to X-ray events is carried out, but the low number of flares prevented an analysis of the direct impact of X-ray flares on the IR lightcurves. However, we find that X-ray detected Class II sources have longer timescales for change in the mid-IR than a similar set of non-X-ray detected Class IIs., Comment: Accepted by AJ 47 (large print) pages including 20 figures and 4 tables

We present a time-variability study of young stellar objects in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 micron with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project (YSOVAR). We have collected light curves for 181 cluster members over 40 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2-6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color-magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability time scales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer time scales than the X-ray undetected members., Comment: Accepted for publication in The Astronomical Journal. Data from Table 2 is given as a machine-readable file in the source of this arXiv submission

The consequences of anthropogenic climate change are extensively debated through scientific papers, newspaper articles, and blogs. Newspaper articles may lack accuracy, while the severity of findings in scientific papers may be too opaque for the public to understand. Social media, however, is a forum where individuals of diverse backgrounds can share their thoughts and opinions. As consumption shifts from old media to new, Twitter has become a valuable resource for analyzing current events and headline news. In this research, we analyze tweets containing the word "climate" collected between September 2008 and July 2014. Through use of a previously developed sentiment measurement tool called the Hedonometer, we determine how collective sentiment varies in response to climate change news, events, and natural disasters. We find that natural disasters, climate bills, and oil-drilling can contribute to a decrease in happiness while climate rallies, a book release, and a green ideas contest can contribute to an increase in happiness. Words uncovered by our analysis suggest that responses to climate change news are predominately from climate change activists rather than climate change deniers, indicating that Twitter is a valuable resource for the spread of climate change awareness., Comment: 11 pages, 10 figures

The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope observing program obtained the first extensive mid-infrared (3.6 & 4.5 um) time-series photometry of the Orion Nebula Cluster plus smaller footprints in eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264, L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C). There are ~29,000 unique objects with light curves in either or both IRAC channels in the YSOVAR data set. We present the data collection and reduction for the Spitzer and ancillary data, and define the "standard sample" on which we calculate statistics, consisting of fast cadence data, with epochs about twice per day for ~40d. We also define a "standard sample of members", consisting of all the IR-selected members and X-ray selected members. We characterize the standard sample in terms of other properties, such as spectral energy distribution shape. We use three mechanisms to identify variables in the fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and significant periodicity. We also identified variables on the longest timescales possible of ~6 years, by comparing measurements taken early in the Spitzer mission with the mean from our YSOVAR campaign. The fraction of members in each cluster that are variable on these longest timescales is a function of the ratio of Class I/total members in each cluster, such that clusters with a higher fraction of Class I objects also have a higher fraction of long-term variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color, we find that a star with a longer period is more likely than those with shorter periods to have an IR excess. We do not find any evidence for variability that causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members and field objects combined, at most 0.02% may have transient IR excesses., Comment: Accepted to AJ; 38 figures, 93 pages

The emission from young stellar objects (YSOs) in the mid-IR is dominated by the inner rim of their circumstellar disks. We present an IR-monitoring survey of about 800 objects in the direction of the Lynds 1688 (L1688) star forming region over four visibility windows spanning 1.6 years using the \emph{Spitzer} space telescope in its warm mission phase. Among all lightcurves, 57 sources are cluster members identified based on their spectral-energy distribution and X-ray emission. Almost all cluster members show significant variability. The amplitude of the variability is larger in more embedded YSOs. Ten out of 57 cluster members have periodic variations in the lightcurves with periods typically between three and seven days, but even for those sources, significant variability in addition to the periodic signal can be seen. No period is stable over 1.6 years. Non-periodic lightcurves often still show a preferred timescale of variability which is longer for more embedded sources. About half of all sources exhibit redder colors in a fainter state. This is compatible with time-variable absorption towards the YSO. The other half becomes bluer when fainter. These colors can only be explained with significant changes in the structure of the inner disk. No relation between mid-IR variability and stellar effective temperature or X-ray spectrum is found., Comment: accepted by ApJ, 24 pages, 17 figures

Variability is a signature property of cool young stars, particularly for those surrounded by disks. Traditional single-band time series display complex features associated with accretion, disk structure, and accompanying stellar activity, but these processes are challenging to model. To make progress in connecting observed time domain properties with the underlying physics of young stars and their disks, we have embarked on an unprecedented multiwavelength monitoring campaign: the Coordinated Synoptic Investigation of NGC 2264 ("CSI 2264"). Beginning in December 2011, CSI 2264 has acquired 30 continuous days of mid-infrared time series from Spitzer, simultaneous optical monitoring from CoRoT and MOST, X-ray observations with Chandra, as well as complementary data from a number of ground-based telescopes. The extraordinary photometric precision, cadence, and time baseline of these observations enable detailed correlation of variability properties at different wavelengths, corresponding to locations from the stellar surface to the inner AU of the disk. We present the early results of the program, and discuss the need for further modeling efforts into young stars and their disks., Comment: Astronomische Nachrichten (AN) Vol 334, in press; contribution to proceedings of the 17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun (ed. Strassmeier & Lopez-Morales); 4 pages, 4 figures

We present intermediate-resolution spectroscopic data for a set of dwarf and giant galaxies in the Coma Cluster, with -20.6 < M_R < -15.7. The photometric and kinematic properties of the brighter galaxies can be cast in terms of parameters which present little scatter with respect to a set of scaling relations known as the Fundamental Plane. To determine the form of these fundamental scaling relations at lower luminosities, we have measured velocity dispersions for a sample comprising 69 galaxies on the border of the dwarf and giant regime. Combining these data with our photometric survey, we find a tight correlation of luminosity and velocity dispersion, L \propto \sigma^{2.0}, substantially flatter than the Faber-Jackson relation characterising giant elliptical galaxies. In addition, the variation of mass-to-light ratio with velocity dispersion is quite weak in our dwarf sample: M/L \propto \sigma^{0.2}. Our overall results are consistent with theoretical models invoking large-scale mass removal and subsequent structural readjustment, e.g., as a result of galactic winds., Comment: 15 pages, 11 figures, 1 table. MNRAS, in press