Disk dispersal timescale at low metallicity – A Multiwavelength study of Dolidze25


M. G. Guarcello1, K. Biazzo2, J. J. Drake3, G. Micela1, L. Prisinzano1, S. Sciortino1, F. Damiani1, E. Flaccomio1, C. Neiner4, N. J. Wright5


1: INAF – Osservatorio Astronomico di Palermo, Italy

2: INAF – Osservatorio Astronomico di Roma, Italy

3: SAO Harvard-Smithsonian Center for Astrophysics, USA

4: LESIA – Paris Observatory, France

5: Astrophysics Group – Keele University, England

The dispersal timescale of protoplanetary disks is one of the fundamental parameters we need to constrain in order to understand the planetary formation process [e.g., Helled+2014]. In order to test the disk dispersal timescales in low metallicity environments, we have estimated the disk fraction of the low-metallicity young stellar cluster Dolidze 25.




Metallicity sets important disks properties and thus it can impact disk dispersal timescale. For instance, dust opacity declines at low metallicity, resulting in a more efficient penetration of energetic photons in disks [Gorti & Hollenbach 2009]. This can increases the effectiveness of magnetic instabilities, which are important for accretion [Hartmann 2009] and magnetically-driven winds [Suzuki+2010]. As a consequence of this, disks dispersal may occur faster in low-metallicity environments [e.g. Ercolano+2009].

Observational studies provide contradictory results: Studies of the stellar population of a few low-metallicity star forming regions in the Outer Galaxy found very low disk fractions compared with regions with a metallicity closer to Solar values [e.g., Yasui+2016a,b]. On the other hands, studies of actively accreting Young Stellar Objects in the Magellanic Clouds found more intense and long-lived accretion rates compared with disk in the Milky Way [e.g., de Marchi+2017].



Left panel: Mass accretion rates of stars with disks in Solar metallicity regions (black) and in the low metallicity region around SN1987A in the Large Magellanic Cloud [de Marchi et al. 2010].







Right panel: Disk fraction as a function of clusters age. The black dots are values of clusters with Solar metallicity, red dots with low metallicity in the Outer Galaxy [Yasui et al. 2010]. 



We have selected the disk-bearing and disk-less stellar population of the low-metallicity [Negueruela+2015] stellar cluster Dolidze25 (4.5±0.5 kpc; log(age)=6.2±0.3 [Myrs]), from new Chandra/ACIS-I observations (150 ksec) and archival optical and infrared photometric data. After having carefully discarded all possible contaminants, we selected 424 candidate disk-less members and 220 stars with disks. Our selection is fairly complete between 0.8 and 2 solar masses. The figure on the right shows the combined Chandra/ACIS-I image of Dolidze25, with events in the soft energy band (0.5-1.2 keV) drawn in blue, medium band (1.21-1.99 keV) in green, and hard band (2-7 keV) in red.
















The left panel shows a Spitzer/IRAC [8.0] image of Dolidze25 [Puga+2009], with the ACIS-I field delimited by the black box. Symbols mark the positions of selected candidate cluster members: stars with disks in red, without disks in yellow, spectroscopic members picked up from literature (massive and intermediately massive stars) in blue



We calculated individual stellar parameters from optical color-color and color-magnitude diagrams. After checking for all possible biases, we calculated the disk     fraction of Dolidze 25 (0.34±0.02) in the mass range 0.8-2 solar masses.


In the left panel, the disk fraction of Dolidze 25 (marked with a star simbol) is compared with that of solar metallicity clusters (black dots) and massive clusters (red dots) within 2 kpc from the Sun, and low metallicity clusters in the Outer Galaxy (green dots). The disk fraction in Dolidze 25 is lower than that of coeval clusters with Solar metallicity and it is similar to that of massive clusters where disks are rapidly dispersed by externally induced photoevaporation.


The cluster contains only 5 O stars (1 O6, O7, O7.5, and two O9.7). They produce an intracluster UV field typical of the outskirt of intermediately massive clusters such as NGC6611 [Guarcello+2010], where no effects due to externally induced photoevaporation are observed. The low disk fraction in Dolidze 25  is thus more likely a consequence of a shorter disks lifetime due to the low cluster metallicity.


Our study thus confirms a faster dispersal of the small dust component in protoplanetary disk in low-metallicity environments as probed by excesses in the near-infrared bands.

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