The tricky line of sight towards Cygnus-X: The [DB2001] CL05 embedded cluster as a pilot case
D. de la Fuente, C. G. Román-Zúñiga, E. Jiménez-Bailón, J. Alves, S. Venus
HHighlights on Spanish Astrophysics X, Proceedings of the XIII Scientific Meeting of the SpanishAstronomical Society held on July 16 – 20, 2018, in Salamanca, Spain. B. Montesinos, A. AsensioRamos, F. Buitrago, R. Schödel, E. Villaver, S. Peréz-Hoyos (eds.), 2019
The tricky line of sight towards Cygnus-X: The[DB2001] CL05 embedded cluster as a pilot case
D. de la Fuente , C. G. Rom´an-Z´u˜niga , E. Jim´enez-Bail´on , J. Alves , and S.Venus Instituto de Astronom´ıa, Unidad Acad´emica de Ensenada, Universidad NacionalAut´onoma de M´exico, Ensenada 22860, Mexico Institute for Astrophysics, University of Vienna, T¨urkenschanzstrasse 17, 1180 Vienna,Austria
Abstract
The nearest massive star-forming complex, Cygnus-X, is widely used as a laboratory forstar cluster formation and feedback processes, under the implicit assumption that all itscomponents are located roughly at the same distance. We present a multi-wavelength studyof a 15 (cid:48) × (cid:48) field in southern Cygnus-X, where different components involving clustered starformation are overlapped. Preliminary results indicate that the Berkeley 87 and [DB2001]CL05 clusters are actually located at very different distances, invalidating previous claimsof physical interaction between them. This shows the importance of a careful treatment ofextinction and distance calculations for cluster formation studies, particularly in Cygnus-X. The Cygnus-X star-forming complex is usually regarded as an ideal workbench for under-standing massive star formation as a whole, thanks to its proximity and richness in youngstar clusters and cluster-forming clouds ([11, 18]). This unique combination helps to connectthe small- and large-scale processes that are involved in massive star formation, and allowsto test the effects of feedback from recently formed massive clusters in detail. However, thesestudies have been often performed under the assumption that all the Cygnus-X componentsare located roughly at the same distance (e.g. [10, 15, 16]), despite multiple evidence againstit (e.g. [17, 21, 13]). Furthermore, [21] claimed that interstellar clouds and OB associationsare actually arranged in several layers at different distances, following the direction of theLocal Galactic Arm, which is nearly perpendicular to the sky plane at Cygnus.Unfortunately, distance estimation is particularly problematic for Cygnus-X. Due tothe fact that the line of sight is roughly tangent to the Galactic rotation curve, kinematic a r X i v : . [ a s t r o - ph . GA ] M a r The tricky line of sight: [DB2001] CL05
Figure 1: Infrared (left) and X-ray (right) RGB images covering the [DB2001] CL05 regionand the overlapping part of Berkeley 87. Colors are R = [5 . K , B = J for the infraredimage, and R = 0 . − . . − − l ∼ ◦ ,up to several kiloparsecs ([5]). Morever, Gaia [8] parallaxes are usable only to the extent thatthe target is unextinguished enough to be optically detected, which happens at d (cid:46) Hii regions ([3]); and an embedded cluster, [DB2001] CL05, discoveredindependently by [2] and [4]. The latter seems coincident with a clump of hard X-ray emittersthat was detected by [16]. By assuming that all the components belong to a single star-formingcomplex, [16] interpreted these X-ray features in terms of interaction between winds fromBerkeley 87 massive stars and the ON2 cloud. This scenario was subsequently challenged bythe trigonometric parallax measurement of a ON2 water maser located at northern [DB2001]CL05, yielding d = (3 . ± .
13) kpc [1], three times farther than Berkeley 87 (Cf. [20]).
A 15 (cid:48) × (cid:48) field covering Berkeley 87 and [DB2001] CL05 was observed through the 3.5-meter telescope of the Calar Alto Observatory, Spain. The resulting near-infrared images andphotometry are merged with those from the Cygnus-X Spitzer
Legacy Survey. We also makeuse of archival data from the
Chandra
X-ray Observatory. Fig. 1 displays the correspondingRGB compositions in a subfield covering [DB2001] CL05, where several spectroscopicallyconfirmed massive members of Berkeley 87 show clearly distinct colors. Specifically, the . de la Fuente et al. (cid:48) × (cid:48) OMEGA2000 field,showing the low-reddening population in blue, highly extinguished stars with no intrinsicreddening in green, and highly extinguished objects with extra mid-infrared excess in red.(b) NICEST extinction map for all sources in the OMEGA2000 field. (c) Same as pannelb, but using only sources with high extinction and no intrinsic reddening (i.e. green dots inpannel a). These extinction maps are drawn in a common linear scale where black is set at A V = 3, and white at A V = 25. The colored squares enclose the region displayed in Fig. 1.latter appear bluish in the infrared image, and orange/red (i.e. soft) in X-rays, in contrastto the souces that seem to be part of the [DB2001] CL05 overdensity. This color distinction,also seen as bimodal distributions in near-infrared colors (e.g. the two groups of data pointsabove/below J − K ≈ . Gaia
DR2 [7] to calculate distances for op-tically detectable stars (which do not include the [DB2001] CL05 cluster). A new distancefor Berkeley 87, (1669 ±
12) pc, is obtained as the median of spectroscopically confirmedcluster members (taking spectral types from the literature). Although somewhat higher thanprevious distance estimates (e.g. [19, 20]), this revised value is still much lower than theaforementioned water maser distance measured by [1].
The tricky line of sight: [DB2001] CL05
Figure 3: Declination vs. visual extinction for infrared counterparts of
Chandra detectionswithin the 15 (cid:48) × (cid:48) OMEGA2000 field, shown as open circles whose radii are scaled withK-band magnitude. Pink and green crosses represent spectroscopically confirmed Berkeley87 members, and further candidates listed by [19], respectively.Figure 4: Berkeley 87 confirmed members (squares), primary (circles) and secondary (dia-monds) YSO candidates, and X-ray sources (crosses) over the OMEGA2000 K-band image(excluding the less relevant southern part). Color symbols are objects assigned to the Berke-ley 87 layer (blue) or the ON2 layer (red). . de la Fuente et al. Observational evidence presented in previous sections may indicate that the overlappingyoung star clusters are part of at least two physically unrelated regions, being located at verydifferent distances. Unfortunately, distances cannot be accurately determined beyond theBerkeley 87 layer, and evidence for [DB2001] CL05 and the 3.83 kpc maser belonging to thesame star-forming complex is inconclusive. Consequently, we aim at separating the Berkeley87 and ON2 populations through extinction estimates to individual objects displaying signsof young age – in a chronological sense. Such category includes Young Stellar Objects (YSOs)but also evolved massive stars whose ages are limited to a few Myr. In this regard, X-rayemission, which is expected to be displayed by both YSOs and hot massive stars ([6]), becomesan ideal diagnostic for inhomogeneous young populations.Primary YSO candidates are found using the classical criteria from [9], and a largeramount of secondary candidates are selected through our own method for measuring intrinsicreddening (de la Fuente et al., in prep.). X-ray sources whose infrared counterparts show nointrinsic color excess are considered to be class III pre-main sequence stars, or Berkeley 87members (note that these two options are not mutually excluding).To provide optimal extinction estimates for as many sources as possible, several methodsare combined. First, direct measurement of color excess is performed for stars of knownspectral type. Second, the Rayleigh-Jeans Color Excess (RJCE) method ([14]) is used in thesuitable cases. Finally, the extinction map of 2c (whose validity is checked against sources incommon with the RJCE method) is applied for stars belonging to the high-reddening group(See Sect. 2), including those that show intrinsic color excess (red dots in Fig. 2a). Theextinction results for X-ray emitters are shown in Fig. 3, where several components can beclearly distinguished, and a wide gap between the Berlekey 87 and [DB2001] CL05 layers isevident. The apparent extinction shift within Berkeley 87 is attributed to class III objectsexperiencing residual color excesses that affect RJCE results .Based on Fig. 3, YSO candidates with extinction values A V >
11 are assigned to theON2 layer. Moreover, any objects with signs of young ages (including spectral types) whoseparallaxes are compatible with the Berkeley 87 distance are allocated in the correspondinglayer. The outcome is displayed in Fig. 4. A vast majority of YSO candidates are located inthe ON2 layer, with a strong overdensity at the position of [DB2001] CL05, while Berkeley87 hosts only a few, mainly class III sources in the outskirts. These results are consistentwith two independent clusterings of different evolutionary stage.
In contrast to previous claims, our preliminary results prove that star formation and X-rayemission from [DB2001] CL05 cannot be physically related to Berkeley 87 by no means, sincethese clusters are separated by a long distance (despite the line of sight coincidence). This Note that intrinsically red YSOs in ON2 are not affected by this problem, since their extinction valuesare obtained through a NICEST map where such sources have been excluded from the map creation process.
The tricky line of sight: [DB2001] CL05 case illustrates the importance of a careful treatment of extinction and distance for Galacticstudies of clustered star formation, in order to avoid reaching wrong conclusions about feebackfrom fake neighbors.
Acknowledgments
D. dF acknowledges the UNAM-DGAPA postdoctoral grant. C.R.Z. and E.J.B. acknowledge supportfrom Programa de Apoyo a Proyectos de Investigaci´on e Innovaci´on Tecnol´ogica, UNAM-DGAPA,grants IN108117 and IN109217, respectively. The scientific results reported in this article are basedin part on data obtained from the Chandra Data Archive. This research has made use of softwareprovided by the Chandra X-ray Center (CXC) in the application package CIAO. This work has madeuse of data from the European Space Agency (ESA) mission
Gaia , processed by the
Gaia
DataProcessing and Analysis Consortium (DPAC).