Chemical Abundances of Red Giants in the Open Cluster NGC 2420 from APOGEE ObservationsSouto, D.(1,2); Cunha, K.(2); Mészáros, Sz. (3); Allende Prieto, C. (1,4); Garcia-Hernandez, A. (1,4); Zamora, O. (1,4); Carrera, R. (1,4); García Pérez A. (1,4) and the APOGEE Team.1 - Instituto de Astrofísica de Canarias - IAC; 2 Observatório Nacional - MCT; 3 - ELTE Gothard Astrophysical Observatory; 4 - ULL (Universidad de La Laguna).

Abstract - The open cluster NGC 2420 is relatively old with an age of roughly 3 Gyr. Chemical abundances of 12 elements: O, Mg, Al, Si, K, Ca, Ti, Cr, Mn, Fe, Co and Ni, were derived for a sample of twelve red giants in NGC 2420 to be used as calibrators for the automatic APOGEE Stellar Parameters and Chemical Abundance Pipeline (ASPCAP). The analyzed spectra were obtained with the APOGEE infrared spectrograph, part of the instrument suite on the Sloan Digital Sky Survey (SDSS) 2.5-m telescope. The spectra have a wavelength coverage between 1.5 - 1.7 micron and a resolution of R ~ 22,500. The abundance determination was done in LTE using the MOOG code with spectral synthesis method. We obtain iron abundances indicating that this is a metal poor open cluster with sub solar metallicity, [Fe/H] = -0.24 ± 0.04.

The Sample – We examined a sample of 12 red-giant stars belonging to the open cluster NGC 2420 (Frinchaboy et al. 2013). The spectra analyzed in this study were obtained with SDSS3 / APOGEE-1 Survey (Majewski et al. 2015). APOGEE-1 is a multi-fiber (300 fibers) high-resolution (R~22.500) cryogenic spectrograph observing in the H-band between 1.5 and 1.7 micron (Wilson et al. 2000). The APOGEE-1 Survey observed more than 100.000 stars in 4 years of operation having ended in 2014. SDSS4 / APOGEE-2 is extending the Apogee survey six more years. The open cluster NGC 2420 is one of the calibration clusters for the APOGEE surveys.

Atmospheric Parameters – The effective temperatures of the stars were obtained from an average of the Teffs obtained from photometric calibrations of Gonzalez-Hernandez & Bonifacio (2009) for the V-J, V-H, V-Ks and J-Ks colors. The V, J, H and Ks magnitudes were obtained from UCAC4 and 2MASS catalogues and a reddening of E(B-V) = 0.05 (Anthony-Twarog et al. 2006) was adopted. See Figure 1 left panel for the adopted (J-Ks) calibration. The surface gravities (log g's) were determined from the derived Teff and using Padua isochrones (Bressan et al. 2012) - See Figure 1; Right panel. Our derived parameters are in Table 1 see also Figure 1. Most of our targets have Teff ~ 4800 K and log g ~ 2.9, although we have a cooler red-giant up the giant branch (Teff ~ 4100 K; log g ~ 1.7). The microturbulent velocities were obtained using Fe I lines adopting the same procedure described in Smith et al. (2013).

Methodology – In this work we perform a classical abundance analysis of the APOGEE spectra with the MOOG spectrum synthesis code and adopt the spectral synthesis method, see Figure 2. The model atmospheres adopted were calculated for the APOGEE-1 project (Meszaros et al. 2012). The line list adopted for the calculation of synthetic spectra was developed by the ASPCAP team (Shetrone et al. 2015).

Figure 2: Synthetic spectrum in a region of OH lines.

DR12 versus Manual analysis - The open cluster NGC 2420 was selected in this study to serve as a calibration cluster to the Apogee chemical abundances pipeline (ASPCAP) that is under continuous development by ASPCAP team. One of the goals of this study is to compare the results from a manual analysis with the automatic abundance pipeline ASPCAP. ASPCAP abundances are obtained from automatic matches of APOGEE spectra to Synthetic Libraries described in (Zamora et al. 2015) for a 7(6)-D optimization of (Teff, log g, [M/H], [C/Fe], [N/Fe], [α/Fe] and (ξ). The results from ASPCAP had to be calibrated with literature results from open and globular clusters and seismic results for red-giants observed with Kepler (Meszaros et al. 2013). Figure 3 shows the comparison of the atmospheric parameters derived here with the results from DR12.

Figure 3 - Comparisons of the atmospheric parameters derived manually in this study with the results from DR12 obtained from the APOGEE abundance pipeline ASPCAP. Top left panel shows the comparisons for the effective temperature. Left bottom panel the surface gravity comparisons. The right panel shows the HR diagram for the stellar sample. The DR 12 calibrated results are shown as blue squares; green circles are the DR-12 uncalibrated data. Our results are represented by the red open circles and connect both calibrated and uncalibrated data from DR 12.

The metallicity of NGC 2420 in the Context of Metallicity gradients in the Milky Way - Figure 4 shows the metallicity gradients for a sample of open clusters observed by Apogee (DR12; Frinchaboy et al. 2015).

Figure 4: Top: Metallicity results from DR12 for 29 open clusters in the Milky Way Disk. The result for NGC 2420 from this study is connected with that from DR12 (black line) We show best fitting slopes adopting an arbitrary break at Rg~15 Kpc. The metallicity gradient obtained for Rg < 15 Kpc is steep (-0.06 +- 0.01 dex/Kpc); and for Rg > 15 Kpc is flatter (-0.02 +- 0.02 dex/Kpc). The typical uncertainties in the metallicities are ~ 0.1 dex. Bottom: Alpha element gradients from DR12. The best fitting slope is flat (0.00 +- 0.01 dex/Kpc).

References: Adibekyan, V. et al. 2012, A&A, 545, 15; Anthony-Twarog, B. et al. 2006, AJ, 131, 461; Bensby, T. et al. 2014, A&A, 562, 28; Bressan, A. et al. 2012, MNRAS, 427, 127; Frinchaboy, P. et al. 2013, APjL, 777, 7; Frinchaboy, P. et al. 2015, submitted; Gonzalez-Hernandez, I., Bonifacio, P. 2009, A&A, 497,497; Holtzman, J. et al. 2015, submitted; Jacobson, R. et al. 2011, AJ, 142, 31; Majewski, S. et al. 2015, submitted; Meszaros, S. et al. 2012, AJ, 144, 8; Meszaros, S. et al. 2013, AJ, 146, 133; Pancino, E. et al. 2010, A&A, 511, 19; Shetrone, M. et al. 2015, submitted; Wilson, J. et al. 2010, SPIE, 7735; Zamora, O. et al. 2015, AJ, 149, 17.

Abundance results for NGC 2420 & Galactic trends - The abundance results obtained for NGC2420 are shown in Figure 5. Results for the Milky Way Disk field stars defining the galactic trends are also shown as comparisons. Overall our results agree well with the disk trends although there are some offsets (e. g. Cr, Co and Mn) that need to be investigated.

Figure 1 - In the left panel we have the temperature calibration of Gonzalez-Hernandez & Bonifacio (2009; dotted line) according to the color index (J-Ks). Right panel: the dotted line indicates the isochrones of Bressan et al. (2012) used to obtain the Log g of the sample stars. In both panels the red circles indicate the derived stellar parameters of the sample. As a comparison we show as green triangles the results from the 12th data release (DR12) of SDSS obtained spectroscopically by APOGEE automated pipeline (ASPCAP), which has been calibrated using various star clusters (Holtzman et al. 2015).

Figure 5: Here we show the galact trends as a function of [X/Fe] over [Fe/H] of the cluster NGC 2420 (blue circles) overplotted with field stars of thin and thick disk from Bensby et al. (2014, green triangle) and Adibekyan et al. (2012, red circle).

Comparisons with results from the Literature - Figure 6 shows the comparison of our results with the only two high-resolution abundance studies in the literature. We find a sub-solar metallicity for the open cluster NGC 2420: A(Fe) = 7.21 +/- 0.04 dex, in agreement with that of Jacobson et al. (2011).

Figure 6 – Diagrams [X/Fe]/[Fe/H], Blue circles represent our results for the cluster NGC 2420. The red squares and green triangles are literature results from Jacobson et al. (2011) and Pancino et al. (2010), respectively.