Latest Publication (Oct 2025)

Atmospheric parameters and chemical abundances of young stars with APOGEE — I. Orion star-forming region

In my most recent publication, I present a detailed study of the atmospheric parameters and chemical abundances of young stars in the Orion star-forming region, based on high-quality infrared spectra from the APOGEE-2 survey. The analysis focuses on G-, K-, and M-type stars with ages younger than approximately 10 Myr. We derive precise stellar parameters for a sample of more than 500 stars. For a carefully selected subsample of slow rotators, we also determine chemical abundances for several key elements, including carbon, magnesium, silicon, potassium, titanium, and iron. We find that the Orion complex is chemically homogeneous and exhibits slightly sub-solar abundances, with alpha-to-iron ratios that differ systematically from those observed in nearby, older stellar populations. These results provide new observational constraints on the chemical properties of star-forming regions and offer important insights into the early chemical evolution of the Milky Way.

Boxplot of the [X/H] abundance ratios derived in this work for each Orion group. The box encloses data between the inter-quartile range (IQR) while the whiskers extend from the box to 1.5×IQR. The orange horizontal line is the median value. The total number of stars for each element is within parentheses. Our determinations suggest that Orion is chemically homogeneous.

The Inti survey

The Inti survey provides reliable and precise spectroscopic stellar parameters, chromospheric activity levels, and photometric rotational periods for 70 solar proxies, 46 solar analogs, 13 solar-like stars, and 9 solar twins, identified in the Northern Hemisphere. We used spectra of moderate (R = 11 930 and 31 500) and high spectral resolution (R = 60 000) obtained with different spectrographs. We estimated the stellar parameters through the differential spectroscopic equilibrium relative to the Sun, which allow us to achieve a high internal precision (σ (Teff ) = 15 K, σ (log g) = 0.03 dex, σ ([Fe/H]) = 0.01 dex, and σ (vt) = 0.03 km/s). Check Yana Galarza et al. (2021) for more details.

Hertzsprung-Russell diagram plotted using the evolutionary tracks of Y2 (Yi et al. 2001; Demarque et al. 2004). The left dashed lines represent the evolutionary tracks for 1.05 M⊙ with [Fe/H] from 0.00 to −0.15 dex, while the right dashed lines for 0.95 M⊙ with [Fe/H] from 0.00 to 0.15 dex. The new solar twins and solar proxies are plotted as circles. To differentiate solar twins from solar proxies we also plotted the Ramirez’s solar twin definition in blue dashed lines, i.e. all the circles plus their error bars that fall within this region are considered as solar twins. The solar analogues and the solar-type stars are shown in diamonds and squares, respectively. The colourmap represents the [Fe/H] for all the stars. The Sun’s data are plotted as reference (green solar standard symbol) with its evolutionary track (i.e. dashed line for 1.0 M⊙ and [Fe/H] = 0.0 dex).

Young Stellar Objects

The young stellar objects (YSOs) are stars at an early stage of evolution, and their properties have a dominant impact on planet formation. Characterizing and understanding YSOs help us to improve our knowledge of star and planet formation. I am working on the characterization of the Immersion GRating INfrared Spectrometer (IGRINS) YSO survey.

I am implementing a Markov chain MonteCarlo technique to determine effective temperature, surface gravity, magnetic field strength, and projected rotational velocity simultaneously. The first part of this project, which is on YSOs located in the Taurus-Auriga star-forming region is under review

Low-mass stars

I used H-band IGRINS spectra to determine the effective temperature of ~K8 -- M5 (temperatures between 4000 and 3000 K) stars using absorption line-depths of Fe, OH, and Al and BT-Settl models.

I also found two line-depth ratios that offer a simple but accurate measure of effective temperatures in cool stars that are distance and reddening independent.

Figure 1. Effective temperature (circles) as function of literature spectral type. The solid line is a weighted fourth degree polynomial fit to the median values of the temperatures. The squares and diamond are two literature temperature scales for dwarfs stars.

Figure 2. Effective temperature as function of line-depth ratio. The dashed line represents a linear fit to the data.

More details and results on this project in López-Valdivia et al. (2019)

Solar-like stars

I used low-resolution optical spectra and Lick-like indices and determine the effective temperature, surface gravity, and metallicity in more than 350 G0 -- G3 stars to identify metal-rich stars. The metal-rich stars are stars prone to harbor giant planets.
More details on this project are able in López-Valdivia et al. (2014) and Chávez et al. (2020).

Solar spectrum reflected in the asteroids Vesta and Ceres. The rectangles represents the central bands of the Lick-like indices used. data.

Comparison of the stellar parameters determined in López-Valdivia et al. (2014, diamonds) and Chávez et al. (2020, circles) and those by the Gaia team (Andrae et al. 2018). The color bar represents the surface gravity value.

I am also determined the abundance of Mg, Al, Si, Ca, Ti, Fe, and Ni (López-Valdivia et al. 2017) and lithium (López-Valdivia et al. 2015) in solar-like stars. For both projects I used the Cananea High-resolution Spectrograph (CanHiS) mounted at the Guillermo Haro Observatory 2.1 m telescope (Mexico).

The stellar chemical composition is an important parameter in stellar and Galactic astronomy studies, and, in particular, in exoplanets. In this latter field, different studies have aimed at searching for possible correlations between chemical composition of host stars and the occurrence of exoplanets.

Distribution of lithium abundance with effective temperature. Black filled circles indicate stars in our sample with [M/H]<0.16 dex, red squares show our metal-richs stars ([M/H] > 0.16 dex), and downward triangles mark 3σ upper limits. Grey dots and downward arrows represent Ramírez et al. (2012) determinations and upper limits. The polygon shows the so called lithium desert.

[X/Fe] versus [Fe/H] ratio for our sample (black circles), Neves et al. (2009, grey triangles), Adibekyan et al. (2012, grey stars) and for Hinkel et al. (2014, grey circles). The vertical dashed line indicates the super-metallicity threshold

Aditional projects

Light curves of transiting exoplanets as well as of asteroids.

Ricardo López Valdivia