Multiple episodes of star formation in the CN15/16/17 molecular complex. ††thanks: Based on observations performed at ESO’s La Silla-Paranal observatory. Programme ID 085.D-0780.
Key Words.:ISM: individual: CN15/16/17 – ISM: bubbles – Open clusters and associations: individual: DB10, DB11 – H ii regions
Context:We have started a campaign to identify massive star clusters inside bright molecular bubbles towards the Galactic Center. The CN15/16/17 molecular complex is the first example of our study. The region is characterized by the presence of two young clusters, DB10 and DB11, visible in the near infrared, an ultra-compact H ii region identified in the radio, several young stellar objects visible in the mid infrared, a bright diffuse nebulosity at 8m coming from PAHs and sub-mm continuum emission revealing the presence of cold dust.
Aims:Given its position on the sky (, ) and its kinematic distance of kpc, the region was thought to be a very massive site of star formation in proximity of the Central Molecular Zone. One of the two identified clusters, DB11, was estimated to be as massive as . However the region’s properties were known only through photometry and its kinematic distance was very uncertain given its location at the tangential point. We aimed at better characterizing the region and assess whether it could be a site of massive star formation located close to the Galactic Center.
Methods:We have obtained NTT/SofI deep photometry and long slit band spectroscopy of the brightest members. We have additionally collected data in the radio, sub-mm and mid infrared, resulting in a quite different picture of the region.
Results:We have confirmed the presence of massive early B type stars and have derived a spectro-photometric distance of kpc, much smaller than the estimated kinematic distance. Adopting this distance we obtain clusters masses of and . This is consistent with the absence of any O star, confirmed by the excitation/ionization status of the nebula. No He i diffuse emission is detected in our spectroscopic observations at 2.113m, which would be expected if the region was hosting more massive stars. Radio continuum measurements are also consistent with the region hosting at most early B stars.
Due to the high degree of interstellar extinction, very few young clusters are known in the Galactic Plane at distances larger than 2 kpc from the Sun. Nevertheless, the central region of the Galaxy, with its unique physical conditions, represents a very interesting laboratory for testing star formation theories (1996ARA&A..34..645M). Recent studies give somewhat controversial results on the star formation efficiency (SFE) in the proximity of the Galactic Center (GC): some suggest a SFE similar to the Milky Way Disk (2009ApJ...702..178Y), others imply a reduced SFE towards the GC, based on a Galactic-wide comparison of dense gas tracers and active star formation tracers (2012ApJ...746..117L). These differences highlight the need for further observational constraints on the star formation scenario close to the GC.
Some studies suggest that several hundred clusters are expected to reside along the line of sight towards the inner Galaxy (2001ApJ...546L.101P), but very few of these clusters have so far been identified. Unfortunately, if we look towards the center of the Galaxy, the identification of clusters is complicated by the severe crowding of fore/background stars. Spurious cluster detections are also possible due to the patchy nature of the interstellar medium, which may cause strong spatially varying foreground extinction and consequently a variation in the star counts. Due to the strong extinction along lines of sight within the Galactic Plane, the detection of distant clusters at optical wavelengths is challenging, even for the most massive clusters located towards the center of the Milky Way. The best way to identify new clusters in the inner Galaxy is therefore to use infrared wavelengths, which are less affected by extinction.
Over the past decade, the search for new clusters has gained fresh interest thanks to near infrared surveys such as DENIS (1999A&A...349..236E), 2MASS (Skrutskie:2006uq), UKIDSS-GPS (2008MNRAS.391..136L) and VISTA-VVV (2007Msngr.127...28A; 2010NewA...15..433M). In parallel to these near infrared ground based surveys, mid infrared surveys of the Galactic Plane have been carried out using the IRAC camera on board the Spitzer Space Telescope. 2006ApJ...649..759C; 2007ApJ...670..428C found almost 600 molecular bubbles in the GLIMPSE I and II surveys. Molecular bubbles associated with H ii regions are a tracer of young, massive clusters hosting O-type or early B-type stars. Within the sample identified in the GLIMPSE II survey (), 29 bubbles are associated with H ii regions, meaning that they very likely host massive stars.
We recently started a campaign aimed at characterizing the stellar content of these 29 regions, using a combination of imaging and long slit spectroscopy. In this paper we present the results for CN15/16/17. We observed the region using the SofI near infrared instrument mounted on the ESO-NTT telescope in La Silla, Chile. The observing strategy consisted of a combination of deep imaging and long slit band spectroscopy of the brightest candidate members. By combining photometry and spectroscopy we have been able to confirm the presence of early B type stars and to constrain the region’s distance using their spectral type classification (see Sect. LABEL:sec:fsd).
The CN15/16/17 complex of molecular bubbles is a star forming region (SFR) hosting young stars in different evolutionary phases (see Fig. 1). The region is projected towards the Galactic Center (, ) and was first detected by 2007ApJ...670..428C, by visually searching the inner of the Galaxy, using mid infrared data from the GLIMPSE II survey (2003PASP..115..953B; 2009PASP..121..213C). The Spitzer/IRAC images of the region show a very pronounced diffuse emission in the m channel, originating from PAH emission. Two stellar clusters are associated with the region and are visible in the near infrared. The clusters were first identified by 2000A&A...359L...9D using 2MASS images (Skrutskie:2006uq). One of them (DB11) has already emerged from its parental cloud and therefore its stellar population is detectable in the near infrared. The second (DB10) is still deeply embedded and the high extinction only allows the detection of the brightest sources. 2003A&A...408..127D further studied the clusters using and band imaging also obtained with NTT/SofI. In addition, a third, very deeply embedded SFR is present. This youngest region consists of a group of young stellar objects (YSOs) visible in the Spitzer images and corresponds to the IRAS 17470-2853 source. It is associated with a radio detected Ultra Compact H ii region as well as several methanol masers (1998MNRAS.301..640W).
In Fig. 1, left panel, we show a composite image of the region from our SofI observations. Objects DB10 and DB11 are clearly visible. DB11 is the central, larger cluster, while DB10 is the smaller cluster west of DB11. The Spitzer/IRAC composite image, right panel, is dominated by bright PAHs emission in the m channel. The contours indicate the radio continuum flux at 1.4 GHz from the NRAO VLA Sky Survey (NVSS, see 1998AJ....115.1693C). From the radio contours it is possible to identify two H ii regions, one associated with DB11, and the other with the aforementioned Ultra Compact H ii region, corresponding to the third, deeply embedded, SFR in the complex. In the latter, a group of YSOs can be seen in the IRAC channels, but most of them are too deeply embedded to be detected in the image. In fact, only two are visible as very red sources. Weaker, but still traceable radio emission is also observed at DB10’s position, resulting in an elongation of the contours.
The paper structure is as follows: in Sect. 2 we describe reduction and photometry for the new SofI images, Sect. LABEL:sec:IRACp deals with YSO identification using IRAC photometry, in Sect. LABEL:sec:spec we describe our SofI band spectroscopic observations, which are used in Sect. LABEL:sec:fsd to derive the spectro-photometric distance of CN15/16/17, in Sect. LABEL:sec:submm we use sub-mm data from the APEX111APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory and the Onsala Space Observatory. Telescope Large Area Survey of the Galaxy (ATLASGAL, 2009A&A...504..415S) to estimate extinction towards the complex, in Sect. LABEL:sec:radio we use integrated radio emission from NVSS to put additional constraints on the spectral type of the most massive members in the DB10 and DB11 clusters, the same is done in Sect. LABEL:sec:nebem using the diffuse Br emission from our spectra, this collection of data is used to derive the masses of the DB10 and DB11 clusters in Sect.LABEL:sec:db10db11, we discuss the results and summarize our conclusions in Sect. LABEL:sec:conc.
2 Jhk Imaging and Photometry
Observations were performed on the 27th of June 2010. A summary of the integration times can be found in Table LABEL:tab:obsstr. The integration times (DIT) were chosen to be short enough to avoid saturation of stars brighter than 9 mag in each of the bands. At each position, NINT=10 frames are averaged to produce a single raw frame; NDIT=5 dithering offset positions are used. The total exposure time is DITNDITNINT.
Given the SofI field of view (FoV) of and their projected distance on the sky of , both clusters DB10 and DB11 are covered at each dither position. We spent an equal amount of time on the target as on a nearby field located to the East and to the North from the science field. This control field was observed to build an image of the sky free of the nebulous emission, which characterizes the CN15/16/17 region. Image reduction was performed using a combination of eclipse routines (Devillard:2001fk) and custom-made IDL routines.
Photometry was performed on the reduced frames using the IRAF implementation of the DAOPHOT package (Stetson:1987qy). Due to the bright and highly variable background, especially in the band, the detection of faint sources in some parts of the image was problematic. In order to tackle the problem, we performed spatial filtering of the image using the Fast-Fourier-Transform (FFT, FFTart) method in IDL. After running allstar on the frames once, the task substar automatically removes the detected sources producing an image which consists of three components: the light from the missed sources , the nebular diffuse emission and the residual noise from the bright infrared sky background .
We used the following procedure to reduce the term. In the previous steps we built a model for the stellar point spread function (PSF) using bright, isolated sources in the field. The FFT of the PSF has been used as a high-pass filter to remove the power associated with small wave numbers (i.e. the large-scale variations of the diffuse nebulosity). Spatial filtering consists of multiplying the FFT of the image with the FFT of the PSF, and performing an inverse FFT on this product. In the final image the component is significantly reduced. The high-pass filtering removes most of the power associated with wave numbers , where is the typical length scale of the PSF (for example the full width at half-maximum (FWHM) in the case of a Gaussian PSF).
The spatial filtering does not account for the noise and the Poisson noise associated with the signal. Both of these components are indeed highly variable (i.e. their characteristic length-scale is ). The presence of these two components sets the fundamental detection limit for the faint sources. After applying spatial filtering a second run of the daofind peak-finding routine allows detection of additional sources, without the need of artificially lowering the detection threshold. Only the peak-finding step is run on the processed images. The PSF-fitting task allstar is subsequently performed on the original, unsubtracted and unfiltered image, using an input list obtained by merging the previous detections and the newly found peaks. We iterate this procedure one more time to obtain the final list of instrumental stellar magnitudes in each band.
Figure 2 shows the images before and after the spatial filtering is applied. The upper-left panel shows the original image in the band. The upper-right panel is the result after one PSF-fitting iteration with allstar. The detected sources are fitted and subtracted from the original image. The residual light is coming from the undetected sources, the nebula and the sky background. The lower-left panel displays the subtracted image after spatial filtering and the lower-right panel shows the subtracted nebulosity. Schematically we have that: