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System IMF of the 25 Orionis Stellar Group Genaro Su´Arez1, Carlos Rom´An-Z´U˜Niga1, Juan Jos´Edownes2, Miguel Cervi˜No3, C´Esarbrice˜No4, Katherina Vivas4, Monika G

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System IMF of the 25 Orionis Stellar Group Genaro Su´Arez1, Carlos Rom´An-Z´U˜Niga1, Juan Jos´Edownes2, Miguel Cervi˜No3, C´Esarbrice˜No4, Katherina Vivas4, Monika G System IMF of the 25 Orionis Stellar Group Genaro Su´arez1, Carlos Rom´an-Z´u~niga1, Juan Jos´eDownes2, Miguel Cervi~no3, C´esarBrice~no4, Katherina Vivas4, Monika G. Petr-Gotzens5 1Instituto de Astronom´ıa,UNAM sede Ensenada, M´exico 2 Centro Universitario Regional del Este, Universidad de la Rep´ublica,Uruguay, 3Instituto de Astrof´ısicade Canarias, Tenerife, Spain, 4Cerro Tololo Interamerican Observatory, La Serena, Chile, 5European Southern Observatory, Garching bei M¨unchen, Germany Abstract We present the system IMF of the 25 Orionis stellar group complete down to the planetary-mass domain to the intermediate-mass range (10 MJup - 13.1 M ). We fitted several parameterizations to the system IMF to compare it with that in other star forming regions. We also present the advances of the follow-up spectrocopy using several world-wide facilities. We constructed the system IMF of 25 Ori subtracting Introduction to the mass distribution of the member candidates that of the contaminants from the control field and replacing the The IMF is one of the most important functions in bins where lie the giant and subgiant stars (∼ 1−3M ) by modern astrophysics because it is an essential input for the counts from the candidates after applying the distance many studies. There are a large number of IMF studies criterion (Figure 6). To the resultant 25 Ori system IMF in several stellar populations (e.g. Bastian+ 2010) but its we fitted several parameterizations (Figure 7). behavior is still under discussion, specially in the low-mass domain. Young stellar associations are useful places to study the behavior of the IMF in a wide range of mass. In this work we present the system IMF of the 25 Orionis stellar group (25 Ori) covering its full mass range (10 MJup - 12.9 M ). 25 Ori is an ideal group to carry out this study due to the following properties: i) ∼ 365 ± 47 pc away, ii) Figure 2 : CMD used to select the 25 Ori member candidates as the sources 7 − 10 Myr old, iii) extinction of 0:29 ± 0:26 mag and iv) lying inside the PMS locus (red solid curves) defined working with the empirical ◦ a radius of ≈ 0.7 (Brice~no+2005, 2007, Downes+ 2014, isochrone (red dashed curve) and several effects that broaden it. The dotted Su´arez+2017, Brice~no+2018, Kounkel+ 2018, accepted, and dashed lines represent the completeness limits of the DECam and VISTA Su´arez+2018, in prep.). In 2014, Downes+ determined catalogs, respectively. The 1, 5, 7, 10 and 20 Myr isochrones from Baraffe+ the system IMF of 25 Ori with photometric candidates in (2015) and Marigo+ (2017) are indicated by the brown and purple curves, respectively. The gray points show all the detections towards 25 Ori. the mass range 0.03< m=M < 0:80. In this study we extended the mass range coverage of the 25 Ori IMF down Table 1 : Number of member candidates and contaminants lying Figure 6 : System IMF of 25 Ori (filled points). The mass distributions to the planetary mass domain and including the massive inside the PMS locus in a field of view of 1.1◦ radius after correcting of the member candidates and the contaminants from the control field are represented by the open circles and the crosses, respectively. The vertical lines members of the group. by the spatial coverage, as well as their Ic brightness and mass ranges. are the same as in Figure 4. Origin Sources Ic range m range Photometric Data (#) (mag) (M ) 25 Ori FOV 1709 5.08-25.7 13.1-<0.01 Optical: Control Field FOV 920 5.13-23.3 12.9-0.011 I DECam (PI. G. Su´arez) Besan¸conModel 670 8.07-19.3 4.0-0.023 I CIDA Deep Survey of Orion (CDSO; Downes+ 2014) I UCAC4 (Zacharias+ 2013) Luminosity Function (log m−log m )2 A − c ξ(log m) = p e 2σ2 I Hipparcos (Perryman+ 1997) To construct the LF of our member candidates we used 2πσ mc = 0:27 ± 0:02 M , σ = 0:44 ± 0:03 Near-IR: their Ic magnitudes and assigned them distances and ex- ξ(log m) / m−Γ Γ = −0:74 ± 0:09 , m < 0:30 M I VISTA (Petr-Gotzens+ 2011) tinctions generating random number and recovering the values from the normalized cumulative distributions of Γ = 0:81 ± 0:07 , 0:30 ≤ m=M < 1:0 I 2MASS (Skrutskie+ 2006) Γ = 1:50 ± 0:37 , m > 1:0 M h β i these parameters for a sample of 25 Ori spectroscopically ξ(log m) / m−Γ 1 − e−(m=mp) 4 Spatial Completeness and Photometric Sensitivities confirmed members (Figure 3). We did 10 repetitions for Γ = 1:29 ± 0:18, mp = 0:39 ± 0:09 M , β = 2:03 ± 0:21 in a FOV of 1◦ Radius Around the 25 Ori Overdensity. each member candidate and contaminant. Survey Phot. Area Satur. Comp. Satur. Comp. Figure 7 : Lognormal (dash-dotted curve), triple power law (dotted lines) Band [%] (mag) (mag) (M )(M ) and tapered power law (dashed curve) functions fitted to the system IMF of 25 DECam Ic ≈ 70 16.0 22.25 0.16 0.013 Ori. As a reference, the Salpeter (1955) slope is indicated with the gray solid CDSO Ic 100 13.0 19.75 0.86 0.020 line. The vertical lines are the same as in Figure 4. UCAC4 I 100 7.0 14.75 6.33 0.340 c The IMF for the very LMS and BD domain has a slope Hipparcos I 100 <5.0 | >13.5 | c a little steeper than that in other regions like Pleiades (Moraux+ 2003), Blanco 1 (Moraux+ 2007), σ Ori (Pe~na VISTA J 100 12.0 20.25 0.85 <0.010 Figure 3 : Normalized cumulative distributions of the distances (left panel) Ram´ırez+2012) and RCW 38 (Muˇzi´c+2017), but is con- 2MASS J 100 4.0 16.25 19.3 0.028 and extinctions (right panel) of 334 spectroscopically confirmed members of 25 Ori used to assign these parameters to the samples of member candidates and sistent with Collinder 69 (Bayo+ 2011). The peak of the contaminats. best fitted lognormal is consistent with most of these re- gions but with a smaller σ, similar than for the ONC (Da We worked with the LFs of the member candidates Rio+ 2012). The peak of the fitted tapered power law is and contaminants defined by the median of each M bin. Ic higher than that from De Marchi+ (2010) but, within the To the M distribution of the member candidates we sub- Ic uncertainties, both parameterizations are in agreement. tracted that of the contaminants from the control field and For high-masses the IMF is consistent with the Salpeter replaced the bins where lie the giant and subgiant stars (1955) slope (Γ = 1:35), though for the highest masses this (∼ 1−5 mag) by the distribution of the 25 Ori candidates is based on small-number statistics. after applying the distance filtering (Figure 4). Follow-up Spectroscopy We have an ongoing spectroscopic survey to determine memberships and construct the system IMF of 25 Ori with a statistically complete sample of confirmed members in the whole mass range. Part of this survey is presented in Su´arez+(2017). Figure 1 : Spatial distribution of our photometric candidates. The dash- dotted circle indicates the DECam field of fiew covered with the array of CCDs (brown boxes). The dashed and long dashed circles show the 25 Ori area (0:7◦ radius; Brice~no+2018) and the 25 Ori overdensity (0:5◦ radius; Downes+ ◦ ◦ 2014), respectively, centered at αJ2000 = 81:2 and δJ2000 = 1:7 (white open circle). The gray background map indicates the density of LMS and BD pho- Figure 4 : LF of 25 Ori defined by the median values of each bin (filled tometric candidates of Orion OB1a in 10'x10' bins (Downes+ 2014). The points). The uncertainties correspond to 1 σ. The open circles and the crosses white star symbol shows the position of the 25 Ori star. indicate the MIc distributions of the 25 Ori member candidates and of the contaminants in the control field, respectively. The dotted and dashed lines ∼ 80% complete Member Candidate Selection indicate the Hydrogen- and Deuterium-burning limits, respectively. We defined the PMS locus in the Ic vs Ic−J diagram us- System Initial Mass Function ing the empirical isochrone traced by previously confirmed To obtained the masses of the member candidates members in 25 Ori and considering the main uncertainties and to assing a mass to the contaminats we used the 7 and effects that broaden this isochrone. The sources lying Myr isochrones from Baraffe+ (2015) and Marigo+ inside the PMS locus were selected as photometric member (2017) models (Figure 5 and Table 1). Figure 8 : Current status of our spectroscopic survey using several world- candidates (Figure 2 and Table 1). wide facilities. To remove the expected galactic and extragalactic con- Figure 5 : Mass-M relation Ic This project acknowledges support from program tamination from our candidate sample we worked with a used to estimate the masses. UNAM-DGAPA-PAPPIT IN108117, M´exico. control field at the same galactic latitude of 25 Ori and se- The red curve corresponds to the lected the sources lying inside the PMS locus (Table 1). We 7 Myr isochrones from Baraffe+ References (2015) for masses lower than Baraffe, I. et. al. 2015, A&A, 557, A42 Marigo, P. et. al. 2017, ApJ, 835, 77 checked that, for the bright Ic range, this contamination is Bailer-Jones, C.
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