3.5.1
Comparison with Literature
This work presents a detailed UV spectral synthesis for HD72660. It is important to com- pare the outcome of this new approach with previous work. The results of this study are listed in Table 3.2, and shown graphically in Figure 3.28. In this project, we investigated the abundance of 32 elements covering atomic numbers from Z=6-82. We attempted to derive an abundance for every single element that we investigated but in eleven cases, we had to set- tle for an upper limit. These elements include: Nitrogen (–4.7, see §3.4.2), Oxygen (–2.5, see§3.4.3), Scandium(–9.5, see§3.4.10), Zinc (–6.7, see§3.4.19), Ruthenium (–10.5,
see§3.4.23), Rhodium (–10.0, see§3.4.24), Barium (–9.65, see§3.4.26), Osmium (–12.0, see
§3.4.28), Platinum (–9.5, see§3.4.29), Gold (–9.0, see§3.4.30), Lead (-10.0, see§3.4.32).
The upper limits we find for O, Zn, Rh, Pt, Au are higher than solar whereas that of N, Sc, Ru, and Os are lower than solar and exceptionally Ba, Pb are almost solar (see Figure 3.28).
There are two elements, sodium and neodymium, that have been studied in the optical spectra [34] and their abundances are listed in table 3.1. In case of sodium, the majority of the transition that occur in the wavelength range studied here are in the form of Naiii, Naiv, Navi, and Navii. However, their excitation energies range from 51 eV to 143 eV which is very high for them to occur in our spectrum. The element neodymium does not have any useful transitions in our wavelength window.
Table 3.1 provide a list of elemental abundances found in the literature for this particular target. Most of these abundance that have been reported before are almost solar (see Figure 3.28). Aside from sodium and neodymium, there are 13 elements that are revisited in this work: C, N, O, Mg, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba.
Our results show that carbon is the only element for which we find a sub-solar abundance. We find a higher abundance for Ti, Cr , Fe , Ni, Zn, Y compared to their solar value which is almost equal to their literature values. We find that the abundances of Mg , Si, Ca are almost equal to their literature or solar value. For barium and nitrogen we already described the upper limits above.
There are 12 new elements that we investigated in this work with detailed synthesis for the first time and managed to find a value for their abundance. They include the following: Al, P, S, V, Mn, Co, Cu, Zr, Mo, Sn, Yb, Hg. We find that all of these element have abundances higher than solar except for phosphorous which is less than solar. Sulfur also has a very large error bar so it is rather difficult to comment on whether or not it is higher or lower than solar value.
The purpose of this project, as described in §3.1, was to revisit the abundance analysis and spectrum synthesis in the UV. The task has been accomplished and our result, on average, suggest slightly higher abundances compared to the results of previous studies.
The high abundance that we find for elements such as mercury, together with an atmo- spheric excess of elements such as manganese, yttrium, zirconium, platinum, suggest that this star may belong to a category of stars known as “mercury-manganese (Mn-Hg)” type star.
On the other hand, our results clearly indicate the presence of lanthanides which are a series of chemical elements with atomic number 57 through 71. They are chemically similar to scan- dium and yttrium, and they are also referred to as “rare earth elements”. Our results include a few of these rare earth elements and they show a very high abundance: Y (Z=39, log(nY/nH)=– 7.9), Yb (Z=70, log(nYb/nH)=–9.0) and also from optical windows Nd (Z=60, log(nNd/nH)=– 10.5). Showing an evidence for the presence of lanthanides can put this star into another category known as “Am“ type stars. This can provide a supporting argument for what was sug- gested by [36]: HD72660 may represent a transition object in between a late-B type Hg-Mn star and Am type stars.
3.5.2
Further considerations and future work
One of the important phenomena that needs to be taken into consideration is the presence of stellar winds. One of the numerous stellar wind indicators in a spectrum is blue-shifted excess
at 2576.103 Å, Mgiiat 2802.705 Å, and 2795.528 Å, Siii1808.0126, 1816.928, 1817.45 Å, and they appear to be the strongest such lines present in regions for which we have data and that we can model confidently. All of these lines are modeled reasonably well (see Figures 3.14, 3.4, 3.6, respectively) and neither one of them show an excess blue wing absorption. Modeling with a data covering a larger wavelength window, could further clarify this situation. In any case, evidence of detectable (mixed) stellar wind in this star is absent.
In a calm atmosphere of a slowly rotating star like HD72660 (vsini∼6 km s−1 ), some heavier elements sink under the force of gravity through the ambient hydrogen and they drift downwards into the stellar interior. This process is known as “diffusion”. However, in some cases like HD72660, we observe a large abundance of heavy elements and lower abundance of lighter elements in the atmosphere. This suggests that there must be another phenomena to counteract the effects of downward diffusion. This kind of abundance anomaly can occur when radiation pressure from the stellar interior, reverses the downward drift of the low-abundance elements. This phenomenon is known as “radiative levitation” which can operate particularly effectively in slowly rotating stars.
The over-abundance of heavy elements we observe in the atmosphere of HD72660 can be potentially explained by radiative levitation in this star. In a way, it mirrors the phenomenon observed by Hill & Landstreet [8], that even most normal but slowly rotating A0V stars seem to have overabundance of some heavy elements such as Zr, Ba and La.
The evolution on the main sequence and later stages, are greatly influenced by the struc- ture and processes such as separation and complex mixing that occur in the stellar interior and envelopes, thus study of chemical abundances in a star can provide powerful tools to investi- gate these processes. Obtaining accurate measurements of the relative abundances of as many elements as possible is an essential first step to interpreting the information content of stellar chemistry. In this work we show that spectrum synthesis in the ultraviolet wavelength window makes a useful contribution to understanding sharp-line A-type stars such as HD72660