- ID:
- ivo://CDS.VizieR/J/ApJ/747/51
- Title:
- Lagoon Nebula stars. I. Rotation periods
- Short Name:
- J/ApJ/747/51
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have conducted a long-term, wide-field, high-cadence photometric monitoring survey of ~50000 stars in the Lagoon Nebula H II region. This first paper presents rotation periods for 290 low-mass stars in NGC 6530, the young cluster illuminating the nebula, and for which we assemble a catalog of infrared and spectroscopic disk indicators, estimated masses and ages, and X-ray luminosities. The distribution of rotation periods we measure is broadly uniform for 0.5days<P<10days; the short-period cutoff corresponds to breakup. We observe no obvious bimodality in the period distribution, but we do find that stars with disk signatures rotate more slowly on average. The stars' X-ray luminosities are roughly flat with rotation period, at the saturation level (log L_X_/L_bol_{approx} -3.3). However, we find a significant positive correlation between L_X_/L_bol_ and corotation radius, suggesting that the observed X-ray luminosities are regulated by centrifugal stripping of the stellar coronae. The period-mass relationship in NGC 6530 is broadly similar to that of the Orion Nebula Cluster (ONC), but the slope of the relationship among the slowest rotators differs from that in the ONC and other young clusters. We show that the slope of the period-mass relationship for the slowest rotators can be used as a proxy for the age of a young cluster, and we argue that NGC 6530 may be slightly younger than the ONC, making it a particularly important touchstone for models of angular momentum evolution in young, low-mass stars.
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- ID:
- ivo://CDS.VizieR/J/ApJS/209/32
- Title:
- Probable young stars in the MYStIX project
- Short Name:
- J/ApJS/209/32
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Massive Young star-forming complex Study in Infrared and X-rays (MYStIX) project requires samples of young stars that are likely members of 20 nearby Galactic massive star-forming regions. Membership is inferred from statistical classification of X-ray sources, from detection of a robust infrared excess that is best explained by circumstellar dust in a disk or infalling envelope and from published spectral types that are unlikely to be found among field stars. We present the MYStIX membership lists here, and describe in detail the statistical classification of X-ray sources via a "Naive Bayes Classifier."
- ID:
- ivo://CDS.VizieR/J/MNRAS/372/661
- Title:
- XMM view of NGC 6231: OB stars
- Short Name:
- J/MNRAS/372/661
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this second paper of the series, we focus on the X-ray properties of the OB star population of NGC 6231. As a tool for our analysis, we performed a literature-based census of the OB stars in the field of view. More than one hundred objects are found, among which 30% can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. [...] Among B-type stars, the detection rate is only about 25% in the sub-type range B0-B4 and remains mostly uniform throughout the different sub-populations while it drops significantly at later sub-types. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the emission being rather associated with a physical (in a multiple system) PMS companion. However, we still observe a correlation between the bolometric luminosity of the B stars and the measured X-ray luminosity. [...]
- ID:
- ivo://CDS.VizieR/J/A+A/454/1047
- Title:
- XMM view of NGC 6231 open cluster
- Short Name:
- J/A+A/454/1047
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of an X-ray campaign towards the young open cluster NGC 6231. The XMM-Newton observations, of a total duration of about 180 ks, reveals that NGC 6231 is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 610^-15^erg/s/cm^2^ but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 2.5 or 3-arcsec according to the optical/IR catalogue used. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin^2^ and a core radius close to 3.1-arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects -- corresponding to the early-type stars of the cluster -- most of the sources are relatively faint (~5x10^-15^erg/s/cm^2^) with an energy distribution peaked around 1.0-2.0keV.
- ID:
- ivo://CDS.VizieR/J/ApJ/809/161
- Title:
- X-ray obs. and membership probabilities of M37
- Short Name:
- J/ApJ/809/161
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (P_rot_). M37 was observed almost continuously for five days, for a total of 440.5ks, to measure stellar X-ray luminosities (L_X_), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with P_rot_, 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, R_0_=P_rot_/{tau}, where {tau} is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, L_X_/L_bol_, to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500Myr. We find that fast rotators, for which R_0_<0.09+/-0.01, show saturated levels of activity, with log(L_X_/L_bol_)=-3.06+/-0.04. For R_0_>=0.09+/-0.01, activity is unsaturated and follows a power law of the form R_0_^{beta}^, where {beta}=-2.03^+0.17^_-1.14_. This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4-1.3M_{sun}_, P_rot_ in the range 0.4-12.8 days, and L_X_ in the range 10^28.4-30.5^erg/s. Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of ~500Myr.
- ID:
- ivo://CDS.VizieR/J/ApJ/677/401
- Title:
- Xray properties of protostars in ONC
- Short Name:
- J/ApJ/677/401
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The origin and evolution of the X-ray emission in very young stellar objects (YSOs) are not yet well understood because it is very hard to observe YSOs in the protostellar phase. Using COUP data, we study the X-ray properties of stars in the ONC in different evolutionary classes: luminosities, hydrogen column densities N_H_, effective plasma temperatures, and time variability are compared to understand if the interaction between the circumstellar material and the central object can influence the X-ray emission. We have assembled the deepest and most complete photometric catalog of objects in the ONC region from the UV to 8um using data from the HST Treasury Program; deep and almost simultaneous UBVI and JHK images taken, respectively, with WFI at ESO 2.2m and ISPI at CTIO 4m telescopes; and Spitzer IRAC imaging. We select high-probability candidate Class 0-I protostars, a sample of "bona fide" Class II stars, and a set of Class III stars with IR emission consistent with normal photospheres. Our principal result is that Class 0-Ia objects are significantly less luminous in X-rays, in both the total and hard bands, than the more evolved Class II stars with mass larger than 0.5M_{sun}_; the latter show X-ray luminosities similar to those of Class 0-Ib stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/725/2485
- Title:
- X-rays from the star-forming complex W40
- Short Name:
- J/ApJ/725/2485
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The young stellar cluster illuminating the W40 HII region, one of the nearest massive star-forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260pc and 600pc, are used in the paper.