Cygnus OB2 Region Research Articles

High-energy cosmic rays and gamma-rays from star clusters: the case of Cygnus OB2

ABSTRACT We investigate the acceleration of cosmic rays at the termination shock that results from the interaction of the collective wind of star clusters with the surrounding interstellar medium. The solution of the transport equation of accelerated particles in the wind-excavated cavity, including energy losses due to CR interactions with neutral gas in the bubble, shows several interesting properties that are discussed in detail. The issue of the maximum energy of the accelerated particles is discussed with special care, because of its implications for the origin of Galactic cosmic rays. Gamma-ray emission is produced in the cavity due to inelastic pp scattering, while accelerated particles are advected downstream of the termination shock and diffuse at the same time. Both the spectrum and the morphology of such emission are discussed, with a comparison of our results with the observations of gamma-ray emission from the Cygnus OB2 region.

New members of Cygnus OB2 from Gaia DR2

ABSTRACT Due to the high quality of Gaia DR2 stellar parallaxes and proper motions, we can study the Cygnus OB2 region up to G = 17.5 mag in circular region centred at (l, b) = (79.8°, +0.8°) of radius 1°. We detect four overdensities in the Vector Point Diagram. We apply a parametric model of the proper motion distribution and determine their mean proper motions, identify their members, and estimate their distances by analysing the parallaxes of the most probable members. We confirm that one of these regions is Cygnus OB2, whose mean proper motion is (μαcosδ, μδ) = (−2.71 ± 0.02, −4.24 ± 0.02) $\mathrm{mas\, yr}^{-1}$. We identify 2767 astrometric members at an estimated distance of 1683 ± 5 pc. A detailed comparison of these members with 333 photometric and spectroscopic Cygnus OB2 members shows that 33 stars of the last group are not members, 16 of them are O-B stars. 300 members in common lead to estimate a distance to the association of 1669 ± 6 pc. One of the remaining overdensities is the open cluster UCB585 for which we find eight more members, located at ∼1460 pc. The other two groups count 179 and 188 proper motion members which are distributed throughout the region and situated at a distance of ∼1280 pc. The estimated distances of these four groups suggest the existence of different substructures in the Cygnus OB2 region along the line of sight.

Radio observations of evaporating objects in the Cygnus OB2 region

We present observations of the Cygnus OB2 region obtained with the Giant Metrewave Radio Telescope (GMRT) at frequencies of 325 and 610 MHz. In this contribution we focus on the study of proplyd-like objects (also known as free-floating evaporating gas globules or frEGGs) that typically show an extended cometary morphology. We identify eight objects previously studied at other wavelengths and derive their physical properties by obtaining their optical depth at radio-wavelengths. Using their geometry and the photoionization rate needed to produce their radio-continuum emission, we find that these sources are possibly ionized by a contribution of the stars Cyg OB2 #9 and Cyg OB2 #22. Spectral index maps of the eight frEGGs were constructed, showing a flat spectrum in radio frequencies in general. We interpret these as produced by optically thin ionized gas, although it is possible that a combination of thermal emission, not necessarily optically thin, produced by a diffuse gas component and the instrument response (which detects more diffuse emission at low frequencies) can artificially generate negative spectral indices. In particular, for the case of the Tadpole we suggest that the observed emission is not of non-thermal origin despite the presence of regions with negative spectral indices in our maps.

GMRT survey of Cygnus OB2 region

A good fraction of very high energy (VHE; > 100 GeV) sources do not have confirmed counterparts at any of the other branches of electromagnetic spectrum. In the extra-galactic sky, most of the VHE sources with counter-parts are blazars, a sub-class of active galactic nuclei. However, VHE sources with identified counterparts in our Galaxy do have a wide range of categories, ranging such as supernovae remnants, colliding wind binaries, pulsar wind nebulae, micro-quasars, etc. The VHE emission and radio emission due to synchrotron process do occur from the same population of relativistic electrons in several type of sources, so the radio band is the first band to search for possible counterparts to unidentified VHE sources. In addition, the radio spectral index also gives important constraints on compactness and energetics of the electron population. In this work we present a two-frequency radio survey, at frequencies 325 MHz and 610 MHz, of the Cygnus OB2 region with the Giant Metrewave Radio Telescope (GMRT) in order to address several astrophysical problems. The survey covers a wide area (~ 5 square degrees) at sub-mJy sensitivity which helps to discover faint radio sources to several objects for the first time. Also this is the first deep wide area radio survey of the region below 1.4 GHz, and the low radio frequency will preferentially pickup more non-thermal radio sources as compared to high radio frequencies. Here, we present the early results from this survey.

Red supergiants and the past of Cygnus OB2

Context. Red supergiants are the evolved descendants of massive stars with initial masses between 7 and 40 M-circle dot. Their brightness makes them easily detectable in the near infrared, making them useful probes of star formation that occurred several tens of Myr ago. Aims. We investigate the past star formation history of Cygnus OB2, the nearest very massive OB association, using red supergiants as a probe. Our aim is to confirm the evidence, found by previous studies, that star formation in the Cygnus OB2 region started long before the latest burst that gave rise to the dense aggregate of early O-type stars that dominate the appearance of the association at present. Methods. Near-infrared star counts in the Cygnus region reveal moderate evidence for a peak in the areal density of bright, reddened stars approximately coincident with Cygnus OB2. A total of 11 sources are found within a circle of 1 degrees radius centered on the association, of which 4 are non-supergiants based on existing observations. Near-infrared spectroscopy is presented of the remaining seven candidates, including four that have been already classified as M supergiants in the literature. Results. We confirm the presence of seven red supergiants in the region and argue that they are probably physically associated with Cygnus OB2. Their location is roughly coincident with that of the older population identified by previous studies, supporting the scenario in which the main star formation activity in the association has been shifting toward higher Galactic longitudes with time. Their luminosities are compared with the predictions of evolutionary tracks with and without rotation to estimate the mass of their progenitors and ages. In this way, we confirm that massive star formation was already taking place in the area of Cygnus OB2 over 20 Myr ago, and we estimate that the star formation rate in the latest 6 Myr represents a six-fold increase over the massive star formation rate at the time when the progenitors of the current red supergiants were formed. Conclusions. The Cygnus OB2 association has a history of star formation extending into the past for at least about 20 Myr, probably dovetailing with the general history of star formation in the region that gave rise to other associations like the neighboring Cygnus OB9. The sustained massive star formation history also argues for a long lifetime of the giant molecular complex from which Cygnus OB2 formed, whose remnants constitute the present-day Cygnus X complex.

Present and future gamma-ray probes of the Cygnus OB2 environment

The MAGIC Collaboration has provided new observational data pertaining to the TeV J2032+4130 gamma-ray source (within the Cygnus OB2 region), for energies ${E}_{\ensuremath{\gamma}}g400\text{ }\text{ }\mathrm{GeV}$. It is then appropriate to update the impact of these data on gamma-ray production mechanisms in stellar associations. We consider two mechanisms of gamma-ray emission, pion production and decay (PION) and photoexcitation of high-energy nuclei followed by prompt photoemission from the daughter nuclei (${A}^{\ensuremath{\star}}$). We find that while the data can be accommodated with either scenario, the ${A}^{\ensuremath{\star}}$ features a spectral bump, corresponding to the threshold for exciting the giant dipole resonance, which can serve to discriminate between them. We comment on neutrino emission and detection from the region if the PION and/or ${A}^{\ensuremath{\star}}$ processes are operative. We also touch on the implications for this analysis of future Fermi and \ifmmode \check{C}\else \v{C}\fi{}erenkov Telescope array data.

A Preliminary Investigation of the Diffuse Interstellar Line at 8621 Å

ABSTRACTWe have obtained high‐resolution spectra at the Dominion Astrophysical Observatory, the Calar Alto Observatory, and the European Southern Observatory of hot stars in the near‐infrared region to study the diffuse interstellar band (DIB) at 8621 Å. Field stars as well as selected members of the Perseus spiral arm, the ρ Ophiuchi complex, and the Cygnus OB2 association were observed and the equivalent widths of the band measured, as well as interstellar K i absorption at 7699 Å. In total we measure the equivalent width of the 8621 band in 64 stars. In a series of figures we show the correlations of the DIB with reddening, polarization, K i, and strengths of other DIBs at 5780, 5797, and 6613 Å. The quality of the correlations are discussed by means of the Spearman rank correlation test. Good correlations are found with reddening and, among the other DIBs, 8621 correlates best with 5780. By examining the three special regions mentioned above we confirm the weakness of the 8621 DIB in the hot stars of the ρ Oph star‐forming region. In the Perseus spiral arm region we find that the DIBs are formed largely in foreground clouds but not in the gas that appears to have been blown away from the young massive stars of the Perseus arm. In Cyg OB2 we find that the equivalent widths of the 8621 line lie above the linear correlation of equivalent width with E(B - V) and their strength does not vary as strongly with reddening. The heavily reddened Cyg OB2 star 12 is the most extreme example of the relative insensitivity of the 8621 equivalent width to reddening in the Cygnus OB2 region.

INTEGRAL-ISGRI observations of the Cygnus OB2 region

Aims.We analyze INTEGRAL-ISGRI data in order to probe the hard X-ray emission (above 20 keV) from point sources in the Cyg OB2 region and to investigate the putative non-thermal high-energy emission from early-type stars (Wolf-Rayet and O-type stars). Among the targets located in the field of view, we focus on the still unidentified EGRET source 3EG 2033+4118 that may be related to massive stars known to produce non-thermal emission in the radio domain, and on the wide colliding-wind binary WR 140.

Deep radio images of the HEGRA and Whipple TeV sources in the Cygnus OB2 region

Context. The modern generation of Cherenkov telescopes has revealed a new population of gamma-ray sources in the Galaxy. Some of them have been identified with previously known X-ray binary systems while other remain without clear counterparts a lower energies. Our initial goal here was reporting on extensive radio observations of the first extended and yet unidentified source, namely TeV J2032+4130. This object was originally detected by the HEGRA telescope in the direction of the Cygnus OB2 region and its nature has been a matter of debate during the latest years. The situation has become more complex with the Whipple and MILAGRO telescopes new TeV detections in the same field which could be consistent with the historic HEGRA source, although a different origin cannot be ruled out. Aims. We aim to pursue our radio exploration of the TeV J2032+4130 position that we initiated in a previous paper but taking now into account the latest results from new Whipple and MILAGRO TeV telescopes. The data presented here are an extended follow up of our previous work. Methods. Our investigation is mostly based on interferometric radio observations with the Giant Metre Wave Radio Telescope (GMRT) close to Pune (India) and the Very Large Array (VLA) in New Mexico (USA). We also conducted near infrared observations with the 3.5 m telescope and the OMEGA2000 camera at the Centro Astronomico Hispano Aleman (CAHA) in Almeria (Spain). Results. We present deep radio maps centered on the TeV J2032+4130 position at different wavelengths. In particular, our 49 and 20 cm maps cover a field of view larger than half a degree that fully includes the Whipple position and the peak of MILAGRO emission. Our most important result here is a catalogue of 153 radio sources detected at 49 cm within the GMRT antennae primary beam with a full width half maximum (FWHM) of 43 arc-minute. Among them, peculiar sources inside the Whipple error ellipse are discussed in detail, including a likely double-double radio galaxy and a one-sided jet source of possible blazar nature. This last object adds another alternative counterpart possibility to be considered for both the HEGRA, Whipple and MILAGRO emission. Moreover, our multi-configuration VLA images reveal the non-thermal extended emission previously reported by us with improved angular resolution. Its non-thermal spectral index is also confirmed thanks to matching beam observations at the 20 and 6 cm wavelengths.

Unveiling the Cygnus OB2 stellar population with Chandra

Aims. The aim of this work is to identify the so far unknown low mass stellar population of the∼ 2Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members. Methods. We analyzed a 97.7 ksec Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDetect code and were then positionally correlated with optical and near-IR catalogs from the literature. Source events were then extracted with the Acis Extract package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models. Results. We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams we estimate a typical absorption toward Cygnus OB2 of Av≈ 7.0 mag. Although the region is young, very few stars (∼4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ∼13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at leas t 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typ ical X-ray spectral parameters are log NH∼ 22.25 (cm −2 ) and kT∼ 1.35 keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT=3.3 and 3.8 keV, respectively. OB stars are typically softer (kT∼ 0.75 keV). X-ray luminosities range between 10 30 and 10 31 erg s −1 for intermediate- and low-mass stars, and 2.5×10 30 and between 6.3×10 33 erg s −1 for OB stars. Conclusions. The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is essentially related to the magnetic ac tivity of low-mass stars (M/M⊙∼0.5 to 3.0) which display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.

WSRT 21-cm continuum field in Cygnus OB2: in search of more Wolf-Rayet stars

We present a deep look into the Cygnus OB2 region using the WSRT. A 2° x 2° map of the optically highly-obscured region was obtained at 1400 MHz, with a noise level down to 0.2 mJy and an angular resolution of 1″. We will compare the resulting radio point source list with optical and near-IR catalogues, in order to identify stellar wind sources like WR and OB stars.