INTRODUCTION Analyses of star forming regions have helped to shape the standard cartoon of star formation. In this toy model, young stellar objects (YSOs) are first surrounded by envelopes of dust and gas. Observationally, the envelopes of these deeply embedded early protostars reprocess stellar photons and emit the vast majority of their luminosity in the far infrared. Next, the envelope either dissipates or falls onto a circumstellar accretion disk, out of which planets may or may not form. Observationally, this stage, known as the Classical T-Tauri (CTT) phase, corresponds to a spectroscopically visible photosphere veiled UV and IR excess continuum emission due respectively to accretion from disk to star, and by thermal emission from the disk itself. In the final stage, the disk disappears, either through accretion, evaporation, planet formation, or some combination of the three. Observationally, the photometry of an object in this stage, known as the Weak-line T-Tauri (WTT) phase, differs very little from a main sequence object, although its spectrum may still show spectroscopic indicators of youth. However, a correlation between the derived age and the observed evolutionary phase has not yet been strategically tested. Stellar clusters are thought to provide natural laboratories for defining the timescales of the stellar evolutionary phases, as they host a population of similarly aged stars. The age of an object (or group of objects) is determined by comparing the position on the Hertzsprung-Russel (HR) in relation isochronal model tracks. The best current method to estimate the age of stellar clusters is through these comparisons of the observed cluster properties to the HR diagram model tracks. With derived ages of samples of stellar clusters, the evolution of the cluster properties, such as disk fraction, accretion rate, rotation rates, etc., can be mapped as a function of time, which can then be further extrapolated to estimate the age for the evolutionary phases of single stars. Yet, any systematic errors which affect the placement of YSOs on the HR diagrams will bias derived studies of the evolution of cluster and stellar properties. For example, studies of cluster disk fractions as a function of cluster age have been used to estimate the lifetime of circumstellar disks , and hence the time available for planet formation. Once a star’s disk has has been depleted through evaporation and accretion, no material remains for planet-building. shows an exponential decay of cluster disk fractions as a function of cluster age and derives a mean disk lifetime of 3 Myr. However, while the mean behavior of disks seems to be explained by an exponential decay, for an individual star and disk, the variables which predict whether or not the disk will dissipate quickly remain unknown. There is perhaps no better example of the state of our ignorance about the longevity of a disk , and thus the ages corresponding to evolutionary phases, than than the case of TW Hydra. The star TW Hydra (TW Hya henceforth), which is the namesake of the TW Hya association, is the closest (NUMBER???) known CTT and one of the closest YSOs in general . As such, many researchers have taken the opportunity to observe TW Hya with a variety of instruments. Numerous diagnostics (photometry, millimeter interferometery, spectroscopy) of TW Hya suggest that it is still accreting material from a thick circumstellar disk . Despite an abundance of observations, the estimated ages are past what would be expected for the star to have such a substantial disk. Studies of the TW Hya association members estimate ages between 5-20 million years old for the TW Hya association . When TW Hya itself is typed using optical diagnostics, a K7 dwarf most often is the best match . This spectral type, as well as its photometry and HR diagram isochrones, suggest an age of ∼10 Myr for the star – again, quite an old age in terms of understanding the survival of TW Hya’s accretion disk. However, one study of TW Hya in the infrared has suggested that these characteristics could be different. used moderate resolution near-IR spectra to determine a significantly later spectral type of M3. This revised spectral type suggests that TW Hya is younger (6 Myr ???). Adopting the spectral type of , and younger age, is rather appealing in terms of explaining the presence of the disk, yet at the same time requires a better understanding of spectral typing of YSOs. The mismatch between the optical and infrared spectral types is an especially pernicious problem for longitudinal studies of star formation. For field objects, which are typically no longer associated with star-forming material, the infrared spectral types correlate well with optically determined spectral types . For YSOs, the spectral type suggested by an optical spectrum (when available, as optical observations are often not feasible due to a combination of foreground extinction and accretion-shock veiling) does not always agree with the type suggested by the infrared spectrum . Possible reasons that may explain the observed discrepancies may result from a magnetic field, including Zeeman broadening and stellar activity, among other processes. An important piece of this puzzle is the strength of the stellar magnetic field. The magnitude of Zeeman broadening, an effect due to Zeeman line splitting in magnetic fields, increases with increasing wavelength as λ², meaning that optical spectra are less affected by strong magnetic fields than infrared spectra. Thus, any spectral typing using infrared data with spectral lines that are magnetically sensitive will produce a different result when a (strong) magnetic field is present. Strong magnetic fields are also associated with increased stellar activity and star spots. The temperature contrast between the stellar photosphere and the star spot are more severe in optical spectra. In the infrared, the contrast between the star and spot is less pronounced, thus resulting in the derivation of a different effective temperature (and spectral type) than that from optical data. Moreover, the competing effects of Zeeman broadening and stellar activity on the spectral types suggested by infrared spectra increases the uncertainty by which one can claim to know the spectral type (and hence age/mass derived from theoretical models) of these young stars. For TW Hya, suggest the reason for the discrepancy between the optical and infrared derived spectral types is likely due to star spots. Unfortunately this explanation is quite complicated, as the subject of star spots is related to a controversy regarding a possible planet, as well as magnetic field. Radial velocity monitoring of the star TW Hya, which is nearly pole-on , suggests that there may be a planetary companion. Subsequent analysis of the RV signal has been used to rebutte this claim and instead suggests that stellar spots are responsible for any observed RV modulations . — Regardless of the potential existence of a planetary companion, the strength of TW Hya’s magnetic field is not discussed in comparing the TW Hya spectral types, and as discussed above, will also have an impact. — to be written — In this paper, we aim to reveal the role of the magnetic field and reconcile the debated spectral type of TW Hya. We re-evaluate the stellar parameters of TW Hya using a powerful combination of newly obtained infrared spectra over the entire H and K-bands with high spectral resolution needed to observationally discern the magnetic effects and a spectral synthesis code that is capable of making the inclusion of magnetic fields widely possible. We use MoogStokes, a custom driver for the MOOG spectral synthesis code which computes the emergent spectrum of a magnetic star . MoogStokes calculates the Zeeman splitting of an absorption line by using the spectroscopic terms of the upper and lower state to determine the number, wavelength shift, and polarization of components into which it will split for a given magnetic field strength. MoogStokes then computes the polarized radiative transfer of all four Stokes components (I, Q, U, and V) from the bottom of the photosphere to the top. The emergent spectrum is a function of viewing angle as well as orientation of the magnetic field. In order to synthesize the composite spectrum of an unresolved magnetic star, MoogStokes computes emergent spectra at various locations across the stellar disk, applies the effects of rotational broadening and limb darkening, and combines the different emergent spectra to compute the composite spectrum. We present new high signal-to-noise spectra of TW Hya obtained with the Immersion GRating INfrared Spectrometer (IGRINS) instrument that boasts both high resolution ($R={\delta\lambda} = 45000$) and large spectral grasp . IGRINS provides comparable resolving power of CRIRES (a well known high resolution infrared spectrograph at Keck Observatory) at 30 times the wavelength range with no gaps in coverage , allowing the study of orders of magnitudes more spectral lines than was feasible with the previous generation of high resolution infrared spectrographs. Armed with both IGRINS and MoogStokes, we demonstrate that we can use a multitude of spectral lines from infrared spectra to determine the fundamental parameters of stars, which is a promising tactic to wield against clusters of stars in the future. By improving estimates of effective temperatures, surface gravities, rotational speeds, magnetic fields, and infrared continuum excess, this combination of tools promises to be a powerful way to investigate the evolution of these parameters, and the stars themselves, as a function of age.
