In and around the rich association Cygnus OB2

In and around the rich association Cygnus OB2

In and around the rich association Cygnus OB2


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Why Cygnus OB2 and its surroundings?The nearest very massive association (comparable to Westerlund 1, NGC3603...), at 1.5 kpcCygnus OB2 displays examples of the whole upper main sequence zoo:LBVs, Of, B[e], WR…, in addition to main sequence.Lighly obscured stellar component relatively well studied, but census far fromcomplete and very inhomogeneously studiedRelated to nearby associations and nearby star forming regions (Cyg OB9,Cygnus X…)How are superclusters born?What is its star formation history?How has it shaped the interstellar medium in the region?What is its energy input in its surroundings?

Unveiling the stellar component…1953: association discovered by Münch and Morgan1956-1958: further members added by Schulte1991-1995: most thorough study in the visible, region around the centralcluster, by Massey et al.2000: 2MASS starcount-based study by Knödlseder suggests that Cyg OB2may be as rich as a young globular cluster and shows more deeplyembedded extensions

2002: NIR spectroscopy of (J-H), (H-K)-selected members by Comerón et al.provides many new embedded O- and B-star candidates

2003: Hanson confirms that all ofthe lightliest embedded stars fromComerón et al. 2002 are early-type,but most of them evolved B stars,suggesting an older population2008: Drew et al. (IPHAS) find manylikely A stars, mainly adjacent to thevisible cluster, also suggesting anold age2007-2011: XMM and Chandra X-ray imaging by Albacete Colombo etal. and Wright et al. reveal lowermasscomponents, reinforcingevidence for an older population

2008: Comerón et al. find new (J-H), (H-K)-selected members,spectroscopically confirmed, in the outskirts of Cygnus OB2 and adjacentstructures, including Cygnus X

Some interesting members found along the way: WR 142a, a late-type WCstar in the outskirts of Cygnus OB2

The current view on Cygnus OB2Distance well established at ~1.5 kpcEvidence for Cygnus X complex to be at the same distance (welldetermined for masers observer with VLBI)Cygnus OB9 likely to be at the same distance tooTraditionally assumed age of 3 Myr (from the main visibleconcentration) challenged: a significantly older population is presenttoo, with important implications on the energy output history (sufficienttime for massive stars to have exploded as SNe)Most of the stellar census is still pending characterization, even amongits most massive members

New optically visible membersNIR-selection criterion is fine, butprone to contamination:spectroscopically confirmed earlytypestars are ~50% of selectedcandidatesMany previously unidentified, NIRselectedearly-type stars arelightly embedded: the census isvery incomplete, even amongstars accessible to visibleobservationsIncluding B flux helps breaking theconfusion between intrinsically redstars and reddened blue stars. TheNOMAD or UCAC3 catalogs(including 2MASS photometry) makethis possible.

Using the reddening-free criteriaQQBJKJHK= 0.196( B= 0.447( J−−J ) − 0.981( J − K)− 0.098 > 0H ) − 0.894( H − K)− 0.089 < 0about 70% of the contaminants can be removedThis has been applied to an area of 6 x 4 sq. deg. centered on Cygnus OB2,limited to K

Most new stars in the field or in Cyg OB9Very few new stars in the main concentration of Cygnus OB2Many new stars in the low-galactic-longitude part of Cygnus OB2…found mainly because nobody had looked for them there before!knownknownnewnew

T eff -M V diagram for all theselected candidates (newand known) withspectroscopy is consistentwith a single distanceWell-defined main sequencelocusGiants and supergiants wellseparated on the averageSignificant spread in ages

Most young stars in Cygnus OB2, but also some elsewhereExtended Cygnus OB2 (toward lower galactic longitudes) is olderSome young stars in the field too

The confirmation of an old, massivecomponent in Cygnus OB2 is veryimportant to understand its relation withits surrounding medium:Longer time to shape the structureAction of core-collapse supernovaemust have been importantEnergy input into the ISM has actedfor much longer than 3 MyrDerivation of the IMF in Cygnus OB2is complicated by non-coevality: slopevirtually unconstrained by our data

The newly found BD+40 4210 is one of the mostluminous stars of Cygnus OB2!Some characteristics common with LBVs, it is in theS Dor instability strip, but no evidence for pasteruptions…and it is just 5 pc of projected distance from a LBVcandidate nebula, G79.29+0.46

BD+43 3654, a O4If runaway from Cygnus OB2One of the three most massive runaways knownMass and ejection age suggest dynamicalejection mechanism

O stars in isolation?No obvious cluster aroundthe three youngestisolated O stars in thefieldBut very red stars withindications of strong UVexcess in the fields of twoof them: strongly accretingT Tauri stars? Follow-upspectroscopy will tell…

Progression of star formationOverall results suggest that star formation at large scale progresses from lowto high galactic longitude (roughly south to north)Same general direction inside Cygnus OB2: intense star formation therestarted ~10 Myr agoDR21 / W75N is the current site of massive star formation: a OB association inthe making

Globules with ongoing star formation all over theplace: on small scales, star formation historyis more complicated

Observations at long wavelengths, mostly by Spitzer recently and now byHerschel, are telling us much about Cygnus X and star formation there, whichwill be very important for our understanding of the formation of massive starsand the effects of triggered star formation.But the energetics and gas dynamics of the region are dominated by the starsof Cygnus OB2, and this is becoming the most poorly known component of theregion.Globules with ongoing star formation all overthe place

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